The following list is not ordered.

1. I am glad my parents decided to have me. 
2. My parents seem to be glad they had me. 
3. It appears that my parents and my husband's parents derive an intense amount of joy from interacting with their first grandchild, more joy than they receive from any other activity.
4. My family is one of my main sources of happiness and meaning. It will literally die off if I do not have children. (Alternative path: invent longevity technologies? Unfortunately not my skillset.)
5. I am relatively young, healthy, and financially stable. Statistically my child has a very strong chance of leading a positive utility life.
6. You don't have to choose between having a positive impact on the world and having children; you can very much do both. 
   1. During my child's first year of life, I started working on some cool EA Psych/X Risk research. 
   2. During my child's first year of life, my husband published a highly appreciated (objectively measured by upvotes!) AI-alignment literature review and did EtG without getting fired. 
   3. Plenty of EAs doing AMAZING work have kids (e.g., Toby Ord, Julia Wise, Michelle Hutchinson, Peter Singer). 
   4. Nothing at all was accomplished for the first three months after having a baby. Little was done between 3-6 months. 50% productivity was achieved between 6-12 months. At 12 months, things are pretty good. The massive hit to productivity seems to be a 1 X child event, which, amortized over the next 30 years of our expected careers, is not too bad. 
7. After having a child, I feel more connected to humanity in a way that is difficult to describe but is nice. Things I file under this category include:
   1. Having spontaneous, warm conversations with people who would absolutely never speak to me otherwise when I was visibly pregnant. I lived in the same town for 7 years and went from having approximately 1 conversation with strangers a month to ~10 a month. These interactions were all very kind, and made me very happy. (Your mileage may vary if you don't like interactions with strangers.)
   2. Going from feeling vaguely positively towards children to feeling incredibly protective of all human children. Reading about violence against children went from unpleasant to intolerable.
   3. Having more positive conversations with my family/my in laws than ever before. 
       

All this being said: the first three months after childbirth is literally torture for whoever is waking up at night to feed the baby. Plus there is a strong possibility that I and/or my baby would have died if I hadn't given birth in a high quality hospital (we had a prolapsed cord and then a lot of maternal hemorrhaging). So despite all the nice stuff written above, I don't think it's an easy decision to make.

Because I want to.

I've always wanted kids, biological or adopted. I see EA as a question of "how can we use our resources effectively to do good... for the resources we choose to allocate to doing good".

Most people draw the line somewhere between what they want for themselves and what they want for the world. I believe kids will enrich my life and can be done in a way which still enables me to do a lot of good.

Group Rationality and Long-term Investment

Children are important for the future. But who should have them?

First, I imagine a world filled with people like myself. If they have children, these children will be raised by people like myself, who are mostly good and do a decent job. Alternatively, if most or all decided not to have kids, humanity would be far poorer (or extinct) in the future. In game-theoretic terms, the cooperate action is to have children. 

Moreover, if we imagine a world with two classes of people, which I'll call rational altruists and ineffective egoists. In this world, the ineffective egoists have children, due to different values, neglect of the long term, or even carelessness. Those children are unlikely to embrace rational altruist values. Because of this, the rational altruists have a choice about whether to have children to raise with their values, and in the long term, investing in children leads to a better long term world. 

Of course, given the second argument, the reasonable alternative is to propagate values via education and similar. This is perhaps more limited, since education has a limited scope to influence children, but also plausibly more scalable and effective. However, if the world begins to resemble the first proposed world, this counterargument no longer applies. (This is one of several answers.)
 

Partial Selfishness / Sustainability

I love my kids, they bring me joy, and I am partially selfish. Even though I'm happy to give money and work on EA causes, it's not my entire life, and I'm partially selfish. Kids are part of my "being selfish" budget.

More than that, I think it's a reasonable long term decision. Just like I feel that I would be short term and irresponsible to spend 100% of my disposable income on charitable giving, since it would be unsustainable, I feel that abstaining from things that make my life joyful would be a bad decision. At the very least, I am nearly certain that I would have regretted not doing so, and I think that I would have been upset with my past self's values for making that decision.
(This is one of several answers.)

Precommitment to having (more) Children.

Before I was involved in EA, I made decisions. These decisions include commitments, as well as emotional investments and building personal relationships with my wife and family. Given that my emotional and personal commitments cannot be changed without consequences to myself and to others, future decisions must be made in that context. Further, despite my somewhat changed personal values, from either a rule-utilitarian or contractualist perspective, as well as from most non-consequentialist deontological perspectives, it would be morally inexcusable to change certain of the plans to which I am committed. (This is one of several answers.)

Because (i) my wife wanted to have a child and I thought it would strenghten our relationship, (ii) I assumed my child was likely to become a happy person and possibly an EA, (iii) I'd potentially have a very close friend for life.

Because they are the future, literally.

I'm motivated to build a better future, I believe that more happy lives are better.

Our lives are significantly influenced by our parents (nature and nurture) and I think raising good children in an environment where there's an emphasis on caring about others is a good thing for the world.

It's hard for an anti-natal social movement to last the test of time.

I'm planning to have children because I feel excited about the aesthetic of parenthood, it seems wonderful to be able to intimately participate in bringing more life into the world, and many people I respect endorse becoming a parent (1, 2, 3, but the list goes on and on...)

I agree with several of the previous responses, but just to add something I haven't seen mentioned: it is a new / different experience 'outside the convex hull' of anything else. I selfishly enjoy that, because I like to experience new things (travel, changing research focus, ultra-running, etc), but I also believe that all of this gives me a more flexible and broader view of the world and how it can work and what it can contain, in a way that improves my perspective and my thinking. Imagination only goes so far, even for the most creative amongst us.

I think that if kids would bring you joy, you can fit them into your life plan, and your kids are unlikely to grow up to be evil, it's worth having lots!! There's a lot of big problems in the world and not enough time for our generation to solve 'em. I think I want to have 4, and I'm 3 months from being halfway there :)