When I took a test a couple months ago on this topic, I just always told myself: it's all in the name. "Valence shell electron pair repulsion theory"
Think about it:
Electrons are repelled by one another, Molecules are held together because of the strong attraction from one or more atom's valence shells of electrons to the nucleus of another atom. That holds the molecule together, but electrons still want to be as for away from each other as possible. In the case of molecules such as H2O, in which case oxygen has two lone pair of electrons not attached to another atom, those lone pairs push down on the weaker hydrogens, causing a relatively dramatic bent shape.
If you don't think I made it clear enough let me know and I'll explain it better