Well I didn't want to make it a spoiler but i'll provide some background info.
So these 2 people (Owen Wilson , Jennifer Aniston) got married and thought they should have kids. But instead (Owen Wilson) got a dog he named Marley for her birthday. They are both into like news reporting and news paper articles.
So anyways, this dog causes ALOT of trouble, wrecks the house alot, can't be trained well. They always call him the worst dog ever and stuff.
Then they decide to have kids (3 of them) and Marley still messes up stuff (Waking the kids, tripping them, wrecking the house) and Jennifer Aniston stays home with the kids now instead of her job because she doesn't want help or a babysitter. (Everyone's afraid of Marley after this babysitter went nuts). So they want Marley out of the house and to be basically never be seen again. But they regret it of course.
So now, the kids grow up, and the dog is grown. They move so Owen Wilson can fulfill his dreams of being on the New York Times. So they realize the dog is very independent now and sick and always laying down. They take him to the doctor and they realize he's dying from his stomach "Flipping over" from eating too fast. He made it for a little while then they realized he couldn't take it, so they put him to sleep. The family realized they had fun with him and regretted everything. Buried in front yard.
This may not seem sad from what I wrote, but believe me, you will cry if you have a dog and/or a tad sensitive.