Why do people find it so difficult to be alone?

I know so many people that just jump from one relationship to another. So many people think they need a girlfriend or boyfriend to be happy. People that get married and before the divorce is final they are looking to get married again. Like I understand that people crave companionship and want to be accepted. But for me when a relationship ends I just want to focus on myself... Love is selfish, we love another person because all of the good things they do for us. So why don't we love ourselves enough to find a way to be happy alone before we go looking for it elsewhere? When a relationship ends for me I feel like it ended because who I am is different from who that person wanted me to be and it causes us both suffering. If being in a relationship caused pain why is a persons first priority after losing loving to try to find more? I understand to have more good things you have to also have bad things. But why so fast is my real question here?
Why do people find it so difficult to be alone?
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