I love hearing my kids laugh, they're young and sometimes they can laugh so hysterically to the point where almost no noise comes out of them, you know they've gone over the edge of laughing and it makes you laugh just as hard also. You can see their personalities and nearly predict the mannerisms they will possess when they're older. My kids are young but growing up fast and learning new things everyday which is always fun to watch. I also try to imagine what they'll look like when they're adults, if they will be scholars or athletes or maybe both. Everyday I ask them how was school and what did they learn and in turn they ask my how my work was ("how's your work today dad?"), they genuinely enjoy the conversation, they almost enjoy it too much as they can get into fights as to who gets to tell their story first - I wish my wife had that much motivation when talking to me
I can pretty much say that I can't remember who I was before I was a parent because I can't imagine my life without my kids. Even when they're causing trouble to no end, busting my hump, causing scenes in public, crying rivers, making messes, and achieving decibel levels previously undiscovered.
The coolest thing about being a parent is having kids that I love so much, that return my love & offer it unconditionally and make me feel younger everyday when I play with them (and on the flipside make me feel older because my "batteries" aren't holding a charge as much as their's currently do - ever notice a kid can play full throttle non-stop and after a while you have to sit down and take a breather). My kids have added so much to my life, much more than I could have ever added to their lives. It's a great feeling.