What causes early ejaculation?

So last night i had sex with mines and first it was oral and he was doing everything which was really good, when i got down to his dick,it seemed as if he had already cum and i just didn’t think about it, then we won’t all the way but suddenly 3 strokes in he had cum, we went into it again and same thing, we have had sex a couple times now and this has never happened before. What’s the cause of this early ejaculation ?, I don’t wanna ask him cause I’m scared it will ruin his ego or something!