College, a reason enough to be sleep deprived.
Lizzie Hale is rarely not sleep deprived. She can't sleep alone, and even her meager excuse for a teddy bear (it's barely the size of her arm) doesn't help all of the time. Call it self-prescribed insomnia if you will (though self-prescribing doesn't help much when she refuses to go get checked). Because of her constant over-thinking behavior, and her fear for making new friends she kind of cornered herself to accept anyone that would be okay living with her in a cheap apartment, as long as they weren't psycho killers.
William Anderson accepted a last minute offer of an apartment with a girl. A girl. It's not like he expects to get laid, but he's not complaining. Though he doesn't exactly expect Elizabeth Hale either, and he surely doesn't expect for her to slowly become interested in his past, a past he'd rather keep hidden. A past that brings constant nightmares which he quiets by swallowing sleeping pills every night or else he can't sleep at all.
So when a certain bear is donated by accident and Lizzie stops Will from almost overdosing on sleeping pills they decide it's time to change something in their lives.
Hence they come up with an agreement to start sleeping together.