Dentist said i have herpes simplex infection. why did he find that now?

Very common: Remember that not all herpes viruses are the same thing as genital herpes. I'm guessing your dentist saw some small ulcers on your gums, palate or lips. These are commonly evidence of a herpes virus-there is no need to do a culture to confirm this, it is common. You probably picked the virus up a long time ago. Usually these viral lesions have no long term health problems associated with them.

Active lesion.: You probably had an active lesion, which the dentist saw clinically. Herpes viral infections are undetectable by clinical exams unless you have an active lesion. The virus is always present in people who are infected but is usually in remission and does not show symptoms. When you have an active lesion, or sore, the diagnosis is obvious. Have your dentist prescribe Acyclovir which will help.