Your friend told you about a new position
Rebound
Nothing good on TV
It's raining
Celebrate recent weight loss
Prom night
You're already at their place and don't want to have to move your car
Show off new lingerie
Celebrate major victory by favorite team and/or political candidate
Your chance to do it in a specific location (ex: airplane, Burger King bathroom)
To prove we're not in a rut
Forgot to buy a birthday present
Your one chance with a celebrity
Time an egg
Cheer someone up (pity)
Some very protected sex to celebrate the fact that I'm not / we're not / you're not pregnant
Practice
Because he/she is from one of the countries you haven't had sex with a person from yet
"Damn, your calves look good in those cargo shorts" sex
Called/texted the wrong person but she was into it anyway
Because she looks like your superhot cousin and this is the closest it'll ever get to being okay
Because he said he loves you and you're not ready to say it back yet
Your condoms are about to expire
It's getting a little hard
Miscommunication (cheetos sex)
To reinforce good behavior (such as shaving and dental hygiene)
To change the subject
Love