If we have been stung by a bee, we may be wary of bees lest they sting us again and recreate the same pain.
Likewise to be hurt in love can make us wary about allowing ourselves to truly love again for fear of being hurt again.
A woman who is committed to her partner and has her partner cheat on her may feel so betrayed that she decides all men will hurt her; add to this perhaps a father who let her down in childhood and she is well on her way to never trusting a man again and never allowing herself to truly love again.
Equally if a young man is frequently lied to by his mother - perhaps she hides her drinking - when in later years his wife is caught using on-line chat rooms, the lying will be totally destructive to his trust of her and he may also decide all women are not to be trusted.
When the heart has been 'broken' it is difficult to throw caution to the wind and fall deeply in love once more - not impossible, but difficult. There is a fear of truly loving.
Humans learn from their experience and if something has hurt before (like the bee) they are resistant to opening themselves up to the possibility of repeating that experience.
This is a protective mechanism which would have served us well in a more primal setting to avoid complacency amongst the dangers of snakes, wolves and bears. However, when we demonstrate caution in our relationships, we close ourselves off to truly loving and with that we also lose the possibility of ever having a deeply satisfying connection with the one that we love .