Why could be any number of things. However I see two main approaches to deal with it: going again & desensitization.

If you finish too quick, plan accordingly. Perhaps the first round can be a BJ or quickie, followed by a recovery period of 5-10 minutes after which you resume with new stamina.

The other approach is to not blow it by a variety of methods: wear a rubber, use a numbing gel, wear a cock ring, edging, etc. I've tried them all at some point and I feel the cock ring offers the best finish, as it is effective while wearing it, but can be removed at any time for the finish or full sensation.

That being said, going for two rounds can also be really incredible, but does take more time, energy, & patience (from both) so may not always be practical.