He always looks better in your head than he does in reality.
He constantly changes his name.
Your friends think he's stupid, useless, and are either jealous of him or pity you for getting involved with him. Secretly you're pretty sure he's worthless, too.
You don't want your mother to meet him. Ever.
If you talk about him too much, he'll try to abandon you. If you show him off too much to strangers, at least one of them will try to steal him, and he won't fight them off.
The cat wants to pee on him, the dog wants to gnaw on him, and your five-year old constantly spills things like red Kool-Aid on him.
He keeps promising to get a job but usually ends up sitting around the house.
He is not as interesting as he was when you first met.
Most editors think he's unoriginal, unattractive, uninteresting, and flat-out refuse to fall in love with him. Even if they kinda like him, they want to make him change into what they think he should be.
He vanishes whenever the power goes out, your computer fries or you move. Sometimes he doesn't come back.
He keeps waking you up in the middle of the night for no good reason.
He's not as, um, long as you thought he would be.
When he finally gets between the covers you're too tired to do anything fun with him.
You take him from nothing, give him your best, adore him, cling to him, devote most of your free time to him, and in the end you know he's going to leave you and become a total attention slut about whom no one has anything good to say.