One thing that I love about dogs is the fact that, for the most part, they're ALWAYS happy to see their humans. The way they wag their tails when they see you makes you feel like the most important person in the world. And to them, you are.
Another cool thing about dogs is the fact that they'll let you teach them tricks. Ashley loved to fetch things, and he knew how to sit, stay, roll over, heel, and all sorts of other tricks. Try teaching a cat a trick. It's not that cats are dumb, it's just that they're too smart for their own good and they get bored way too quickly for you to teach them tricks. It doesn't take much to make a dog happy. Just give them attention and love.
Dogs are so forgiving; you can abuse them and they will still love you. For instance, my sister used to love "dressing up" poor Ashley. She would put hair clips on his ears and necklaces around his neck, and the whole time Ashley would just sit there with a look of extreme patience on his face. Cats, on the other hand, take a long time to both forgive and forget. I bathe my kittens once a week, because a lot of the family members who come to visit me are allergic to cats. It is a supreme struggle to get them into the bathtub, and I usually emerge with scratches galore and extremely sulky kittens. They fly right under the bed and that's the last I see of them. And they stay mad at me for hours and hours. Ashley was a big dog, and so he would usually get washed outside with our garden hose (frigid water!). He hated it, but he would sit relatively still and he didn't hold any grudges afterwards.
Cats also have a tendency to get in the way, and they seem to think that they own everything. When my parents came to visit me, they stayed with me at my apartment. My dad complained that he felt like a freeway, because if my kittens wanted to get somewhere and he happened to be in the way, they would walk right over him (literally!). Dogs always revere you. Cats only tolerate you when they're in the mood to.