I thought hard about why it would make you so sad. I cried several times during the story, and copiously at the end. One reason I cried is that I'm an old retired guy, and it's difficult to read such stories of youth at this time in my life.
But the other, more universal, reason, I think, is that many of us men are very romantic about love even if we might keep that tendency secret from everyone including ourselves; and for the romantic in us--well, he should have had Aya! She was his first love, and the ideal companion of his soul.
I bet this happens all the time. Someone has a very intense love or crush in their teens, and then they get diverted from that attachment to someone else whom they eventually marry. Quite often that someone else is much better suited to be their companion through the struggles of life. Can anyone doubt that Tsukasa is such a person here? As you said, she's unselfish and always-supportive. In addition she's practical, realistic, and emotionally strong, and she's smart. The fact that she's beautiful ties up the package.
My readings on Japan suggests that it's a "career-mad" society, much more even than the USA. Evidence from manga has shown me that again and again, and so does a good book I'm now reading called "The Japanese Mind". Such shifts of choice of mates from the romantic choice to the practical choice when one is about to go out into the world must be extra-common there. So "Ichigo 100%" is the perfect, representative story of growing up and finding a wife - for that society.
But I bet many a man there, just like some of us readers, remembers his first sweet love all his life, with sadness.