Why am I such a sensitive person?

I am the type of person that gets emotional over everything. I try to not let things get to me, but I can't help it. I am a sap. I cry at movies, I cry when I am upset, and I cry when I am sad. It is hard for me to not take things personally. I guess it is because I have been let down so many times that I try to give people the benefit of the doubt, but when they disappoint me, I can't help but become emotional over it. When people make plans with me, I hold them to those plans. Maybe my expectations of people are wrong. Maybe I look at things differently than I should. Maybe I shouldn't take things so personally. Maybe I should go with the flow and not make plans; that way I wouldn't be disappointed. If I never made any plans and received a call at the right time, then I should just do it. I am not like that. I don't go out often. I don't feel like I have many friends. I have a few friends, and most of the time, they are too busy for me. Most times I can't be penciled in because other people's lives are more eventful than that of my own. I want to do more things. I want to be invited places. I want people to want to hang out with me. I don't want people to feel obligated to call me, or obligated to be my friend. I never want someone to invite me just because someone else says something, but because they want me to be there. I know this probably sounds worse than it is. I just feel so alone. I go home everyday to a wonderful companion, my cat Oliver. He is always excited to see me, and that makes me feel good. He looks forward to me coming home, and when he hears me coming, he is by the door waiting for me. Oliver greets me with a Meow everytime I walk into my apartment. He is the one thing I look forward to when I dread going home and being by myself. I am the only one of most my friends that is single, or doesn't have children, or lives alone. It sucks. Sometimes I am tired of just hanging out with myself.