1. Because you're not in a relationship.
2. Because you are in a relationship and your girlfriend wants to watch it with you.
3. Because you can buy porn and play the porn while you're having sex.
4. It's a time killer.
5. It gets you off.
6. It's free.
7. Your friends are busy and you have time for yourself.
8. You need something new to fantasize about.
9. You're bored.
10. Porn gives you ideas to try in bed. (at least for me, I've definitely discovered some new positions I'd like to try someday.)