You might want to explore all the great reasons for having an only, which is a question posed by another mom today. Read the responses she got here: http://www.mamapedia.com/questions/5929452319845515265
Wanting another baby is an interesting phenomenon. I've felt it myself, and I've watched it in hundreds of friends and acquaintances over the years. The one thing that seems universally true is that it's a biological urge, and has very little actual connection with how happy and fulfilled many women feel. In fact, the inverse is often true: The more children a couple has, especially for the first several years, the more stressed, tense, worried, drained and unhappy the whole family is.
Yes, of course there is love and joy. And the stress factor, the lack of sleep, the medical issues, the endless sibling strains and scheduling and problem-solving and personal demands and child care conundrums and bill-paying, take a toll. It's really impossible to look at the average family of two or more and insist that they don't.
There's also the "thought" connection: What we believe, perhaps have believed all our lives SHOULD make us happy (or not), often blocks us from noticing how satisfying our lives actually are. We often keep chasing dreams that we or others have implanted in us, that on closer inspection just aren't as true as some other possibility. And that could be where your "sooo confusing" comment is coming from.
So before you commit to the enormous task of birthing and raising another person (and remember a "baby" is a "tiny" only briefly – each one is a new person with a whole, complex life waiting to unfold), consider what's best not only for your desires, but for the child you already have, the man to whom you are committed, and your own best interests.
I think you can tell by my comments that I stopped with one. And it was fabulous. I have no regrets at all. (And in spite of what other posters will probably say, there are moms and dads who sincerely regret having another child. For all sorts of reasons. I have heard about many, right here on mamapedia.)