It could be any number of reasons. Here's a few I could think of off the top of my head in no particular order:
1. He could just have excellent control or is experienced in tantric sex. 2. He could be under the influence of some sort of drug or alcohol. Some drugs are specifically for sex, of course, but other recreational drugs can have sexual side-effects. 3. Possibly not enough sensitivity - condoms will cut down on a lot of the feeling for a guy. Of course, if condoms are your source of birth control, I wouldn't give those up. I just wanted to mention that they can reduce sensitivity. 4. He's stressed and worrying about something else. 5. He's really focused on pleasing you. 6. He may hold the belief that the man must wait for the woman to orgasm first. Do you orgasm during your session? If not, he could be psychologically holding back and not even realizing it. 7. It's possible he's not excited by you for whatever reason 8. He's just tired. 9. He "took care of things" prior to your sexual activity. 10. Some sexual positions don't provide as much stimulation for a man - it depends on body shape as well.
From experience, I've had times where I just couldn't reach orgasm even though I am very turned on. I've also had the opposite happen (Damn! So soon?). It really depends a lot on the situation, personal physiology, belief systems, length of time since last encounter, etc.
The best thing to do (assuming you're close) would be to just sit down and talk about it some time when you aren't being sexually active. If you're both open and honest with each other, you may find out a specific reason. Of course, there are many factors including the personality, maturity, beliefs, etc of each of you, so I can't say how it will turn out. But hopefully you'll find something out and either alleviate your fears or improve your love life.