What Causes Premature Ejaculation?

I am a 24 year old guy. I have never been able to have sex because after about 30 seconds of foreplay, I find my self ejaculating. I have tried this with 3 different partners under a two year period and the results are the same. I dont fell tensed when I am aroused or suffer from anxiety which some doctors say. I dont just know what happens. Even while still wearing my clothes. Mere smooching and kissing makes me ejaculate.