I think a lot of people start out this way including myself. As you hit your late 30's I think is when even some hard-core no-kids people start to re-evaluate their thinking.
I still don't have kids but I'm not anti-kids anymore so if one day it happens I'll be ok with it. What changes is that you go around year after year and you start to see things become a bit repetitive. Holidays with the family, celebrations. Your family starts getting smaller. You start to see how much fresh new energy kids bring to life in general. You start to chill out about the crying and hassles that babies bring and not see them as so much of a burden. All of the things that you've done a million times are suddenly new and exciting because you have somebody to share them with and pass off all of your knowledge, show them all the tricks you've learned in life. I didn't really feel any of that until I had nieces, but I really love hanging around with them.
I do agree with you that people flame out because their lives evolve around kids and they lose touch with each other. It can't really be underestimated the amount of energy required to stop that from happening. You always see on TV that marriage takes work but I doubt many people understand what that actually means until they're in it. Little things can creep up on you. Your spouse may feel neglected and you won't even know it until it's gotten to be a serious problem. You might think you're being the perfect parents because you dote on your kids and meanwhile your spouse is lonely and unhappy. You might not even be aware that you're being selfish. Children tend to consume every ounce of energy that the parents have, especially in the early years. That doesn't leave a lot of time or energy left for each other. It really takes a constant effort and work to keep the marriage healthy.
I still don't have kids but I'm not anti-kids anymore so if one day it happens I'll be ok with it. What changes is that you go around year after year and you start to see things become a bit repetitive. Holidays with the family, celebrations. Your family starts getting smaller. You start to see how much fresh new energy kids bring to life in general. You start to chill out about the crying and hassles that babies bring and not see them as so much of a burden. All of the things that you've done a million times are suddenly new and exciting because you have somebody to share them with and pass off all of your knowledge, show them all the tricks you've learned in life. I didn't really feel any of that until I had nieces, but I really love hanging around with them.