The Theory and Practice of Relationships

I am looking for people's thought on relationships. I have been thinking about relationships lately since I am at the point in my life where it is going to take a real effort to meet people with whom I at all compatible.

Are they necessary?

If you are in one what effect does it have on your life?
How did you meet the person?
What keeps the two of you together?
I was wondering how much commonality people have. Was it love at first sight or something that grew more slowly?

If you are not in one why not?
Have you been in one before or have you always been single?