Someone lovable is very easy to love. Kittens and puppies, cute fictional characters, and sweet-faced stuffed animals all tend to be lovable. Clowns? Not so much.
People who are kind and generous to others are also lovable, because they deserve love in return for giving it to others. The adjective lovable comes from love, which has the Old English root lufu, "love, affection, or friendliness."