From the very first chapter on this has been heavily implied. Himmel was a true GIGACHAD, who knew his wish couldn't come true, thus he settled for the present - since the future was unattainable, and would have been ultimately cruel to Frieren."An idyllic dream that one has already abandoned"
Himmel already knew he would never be with Frieren
He chose living his life with no regrets and enjoy the adventure he had with his friends, or rather the most important people in his life.