I think people idealize what having a relationship is like. It is wonderful in the beginning but if you fall in love and the other person doesn't or falls out of love, it hurts on levels that you can't understand. Often enough that you wish you never had met them or experienced the beginning feelings of love. Also, even if you get to the point of marrying, there comes a time when you and/or they stop trying to please you and the real personalities come out. This is a hard time because you are trapped in a forever relationship. This often becomes a time when the "two that became one" go back into being two people that live in the same house only as platonic roomates...or worse, roommates that avoid each other. Anger, frustration, and loneliness become normal feelings. This is when many marriages die and people then just become roommates or get divorced. The emotions that come from divorce include betrayal, rejection, and you begin to question your value as person who deserves to feel love. Being single with a focus on being married often has many of the same feelings without the damage of having a broken heart and the status as being a failure in marriage on top of it.
If your focus is on "poor me, why can't I find a relationship" or "I'd just be happy if I found someone to love me", you are wasting your life mourning something that isn't real. We are to learn to be content with the life God has given us and to use our opportunities to serve him and find our selves as single people, individual in Christ. Anything else makes having a spouse/or dating partner your idol.