Romeo has just killed Tybalt. He could consider himself fortunate to be alive, but a fool for killing his kinsman of one hour.
Romeo has just married Juliet, only an hour before he kills Tybalt.
He could consider himself fortunate to be married to Juliet and and a fool for killing her cousin Tybalt.
Either way, Romeo is in trouble. He is fortunate to be married to Juliet. He is fortunate to be alive, but he is a fool for killing Tybalt.
Ironically, this statement is possibly foreshadowing the end of the story. Romeo is fortunate to have loved Juliet, but he is a fool in the end because he kills himself, unaware that Juliet is only sleeping.