He finishes too quickly.
PE (premature ejaculation) is when a man orgasms too fast for his, or his partner's, satisfaction. This can mean anything from five thrusts to 15 minutes of thrusting, depending on the people involved. Around 30% of men experience PE at some point in their lives with many men ejaculating within two minutes of penetration. Causes include: lessons learned from early masturbation (having to get it over with quickly before being discovered), performance anxiety, not enough regular sex and youth (the younger he is, the more likely it is to happen). Again, it's a touchy subject -- but (bless him) he also feels he's letting you down by not lasting long enough for you to orgasm. (The fact that around 80% of women get their orgasms via his tongue or fingers still hasn't quite sunk in for most men.) That's why you need to make it abundantly clear that you're content and happy; once he knows that, he'll worry less because it takes the pressure off.