1. Children make a fabulous excuse to leave a party that you don't want to be at. "Well, the baby has to take his nap/go to bed now. It's been great." Whether or not this is actually true is beside the point.
2. They are fascinated by the simplest things. My son is much happier with a set of tupperware than some elaborate toy. Makes you think twice about your priorities.
3. You always have somebody to spend your day with.
4. You learn to appreciate nature when you're with your child. My son points to things like flowers, insects and rocks that I never would have noticed had he not been with me.
5. You can become a kid again. To go to the playground and swing on the swings, slide down the slide, jump on a trampoline. Then to hear his infectious laugh - that's the best!