What makes people sad and broken hearted after break-ups especially when they see their ex in a new relationship?

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When people break up whether it is a short a long term relationship most of them become confused. They start to see every person as same as their ex, they become sad and even stressed especially when they see their ex in another relationship. Analyzing the reasons for someone to be sad, we find that, there are no critical reasons as to why these people their happiness is melt down and be cold hearted when they see their formal lovers with other lovers. There is a saying which states that "Friendship is to share and love is hospitality", which means that, when you are friends, you divide equally your problems facing you both and share your strength in solving them and when you are lovers, one among the two of you is ready to carry all the problems and work to solve them all even if it costs death. The question is, what kills the heart between the two after break ups? is it "Friendship" or "Hospitality" ? In my side I choose hospitality that kills the most, what is your views?