At what point do you give up fighting for someone?

At what point does fighting turn to losing?
At what point does one finally recognize and accept a losing battle?
And finally just accept defeat?
At what point does romantic persistent endeavoring become desperate creepy stalking or harassment?
At what point does romantic ideation and the certainty of knowing that one is right for someone and someone is "the one" and is afraid to you what they really feel turn to desperation and terrorizing?
When does one's feelings and intuition end up being wrong about them being able to know what someone else feels?
When does someone stop trying to fight to make someone else's life better who doesn't want them, but whom they know both could be happy with?
To what end does one find these distinctions?

Most importantly...why do we accept the love we think we deserve?