They're cute. In fact, your own child is adorable. Every sigh, every laugh, every motion is bliss. You fall in love with them.
Re-experiencing childhood. Regardless of how good your own childhood was, your child allows you to vicariously relive childhood. Usually it's much better the second time around, when you have all the wisdom of age but can enjoy the energy of youth. Playing with a two-year-old feels like being two years old again.
Learning. Watching your child learn is fascinating and fun. Each new skill or idea or word learned is a little victory. If you watch carefully, you will see that something new is discovered or conquered every single day. Your child will need to learn a lot from you, as he or she starts without much knowledge at all except for perfect sense of his or her feelings. While you are busy teaching your child about practical aspects of the world, your child will be teaching you emotional sensitivity. The active teaching and learning process is rewarding and fulfilling.
Reflection. Everything that you already can do probably take for granted, but your child will remind you that all those skills had to be learned. It reminds you of how far you have come in life, and shows you the sorts of difficulties you probably had when you were your child's age. Your child will also notice things that you have long since filtered out. I didn't realize how many birds there were outside my house until my daughter showed me.
Reincarnation. Your genetic makeup is a small part of your identity. More important are your ideas, beliefs, manners, and stories. All of these things will be taken up by your child and then changed around. Your child will never be a clone of yourself, fortunately, but instead will be a new variation of you and the other people that raise him or her. If you like yourself then this is very satisfying.
Socializing. Every society has a subculture for people who raise children. At every age you can find people also raising a child in a similar situation. You can make many new friends with whom you have a lot in common. The comradely and shared experiences ease some difficulties.
Control. You must not tell other people how to raise their own children. However, if you raise your own child, you get to do it your way, which you presumably believe is the right way. People might disagree with your methods, but if you have thought at great length about it--and you should if you plan on having a child--then you probably have good reasons for all the decisions you will have to make. When you find that your methods are not perfect, which you will, you can change them appropriately.
Adoption. There are lots of children in the world without parents. Adopting a child is an excellent way to greatly improve the life of someone who would have a very hard time otherwise.
Entertainment. Children have lots of energy and can be lots of fun. They develop imaginations, a sense of humor, and lots of excitement. Little kid laughs are infectious. Playing with children is very enjoyable.
Love. You are the center of your child's world, and as much as you may love and need your child, he or she will love you more. Children that are shown love and affection show it back many times over.