Because if you don't, something's wrong with your expectations. Here's the deal:
You get the love you think you deserve.
I'm sure you've heard that before, right? You put up with what you think is fair. Stuff you don't think is fair, you reject. If you don't reject the stuff you don't think is fair, you're not living aligned with your values. Over time, that's going to erode your self confidence. Your system KNOWS when things are out of whack, and the longer it goes on, the harder it will be to break the cycle.
If you don't love the love you're getting, give yourself the love you're missing.
Life's too short to blame, shame, and be resentful.