Loneliness and desperation. Many people tend to feel worthless when they are single hence jumping into relationships without thinking things through.
Friends. Many times people get into relationship because of friends so that they can do things together or do not want to miss out on the game of love when their friends are enjoying it.
Distraction from problems. It is a very common thing to get into relationships to avoid or forget problems. But such relationships fail to survive as they are not based on love.
Pressure to settle down: Relationships for the sake of society often do not stand the test of time.
Need of Physical Intimacy: Desperation for sex can many times result in jumping into the sack with the wrong people.