Because you can't get to sleep
Make-up sex
Break-up sex
Your friend told you about a new position
Revenge
Rebound
Paratrooping / Bangin' for roof
Nothing good on TV
Because you're in a hotel
Curiosity
It's raining
It's halftime
Diet / exercise
Celebrate recent weight loss
You finally get the chance to show your childhood bed some action
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
Stress relief
Just shaved legs
Forgot to buy a birthday present
"Let's Get It On" by Marvin Gaye is playing on the radio
Celebrating the joy of life after a near-death experience
Your one chance with a celebrity
Time an egg
To avoid cleaning, studying, or doing work of any kind
She wants to
It's cold outside
Cheer someone up (pity)
Keeping up with the neighbors
Your roommate is out of town and you can do it on the couch
Some very protected sex to celebrate the fact that I'm not / we're not / you're not pregnant
Practice
They have air conditioning and you don't
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
Breaking in a new apartment
Because he said he loves you and you're not ready to say it back yet
Wingman diving on the friend grenade
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