How come I find myself always getting into serious relationships very quickly?

How quick? I'm not sure how 'quickly' you mean -- but this behavior can stem from many possible causes. You would know your own best -- maybe with the help of a therapist in processing things. Some possibilities might be fear of abandonment or personal insecurity. This can happen in survivors of abuse and also with certain personality disorders. I've seen this pattern also in people with sex addiction.
You have unmet needs. The needs are strong but you don't know what the needs are, so serious relationships then happen quickly.The problem is trying to get the need met without knowing what it is, makes it likely that the relationships wont last and then the cycle begins again. How can you pick out the right characteristics in a person to meet your needs if you don't know what the needs are?