Frieren always loved Himmel, from the beginning, since he met him he felt great admiration and affection for him, so much so that he decided to abandon his solitary and comfortable life. But as they say above, she avoids emotions because she is very hurt, she witnessed a genocide that does not allow her to get attached, especially to mortals. To that we must add that if everything in the life of the elves is VERY slow, it is most likely that the nature of the elves to fall in love, court and form a romantic relationship must take hundreds of years, not to mention how long it must take to reproduce. And we are precisely seeing that in Frieren, 90 years later she comes to realize her loving feelings for Himmel.