People have a hard enough time figuring out what love is, and then there's unconditional love, which some will argue is the only kind of real love.
If you've ever had a moment when you practiced unconditional love, whether spontaneously or deliberately, you probably felt energized and liberated, not drained and burdened. The more often you feel the the former after acting a certain way, the more you're loving unconditionally.
Many people feel a sweep of unconditional love upon the birth of a child. That's not to say everyone does or should, or that you can't feel it otherwise. It just may be a useful way for some people to remember what unconditional love is supposed to feel like.