Getting off

What could be reasons a guy doesn't get off? Like its been going soft in the middle of sex..prob happens two to three times a week out of seven or eight times..I know he loves me and promises it's not me. He said he doesn't know what's wrong with him. Last night I could see it in his eyes that he was sad bc I thought I was the problem. I was thinking about telling him to go to the doc but was curious as to what the problem may be..Any ideas? I'm too impatient to wait to find out