I'm telling you how I see celesaya, or celeszono, whatever you call it. This will be more about Celestia.
In canon, we see her as a confident girl with high self-esteem, and she has every reason to be: she's certainly beautiful, very smart and rich. She lives a near-perfect life. And she's not deprived of attention, at least from men (Hifumi is an example). Most likely, she received a lot of admiration from others even before entering Hope's Peak. But she remained lonely (her most precious thing is literally a cat, guys). Perhaps she pushed people away, trying to show that she didn't need it, or simply not considering anyone worthy enough to be close to her. Celestia has reached such heights, accomplished so much that people usually don't achieve in their entire lives, and she's only about 16 or 17 years old, yet she exists with a lack of warm human love and care. And when Sayaka appears in her life, genuinely interested in Celestia as a person, everything falls into place. Caring, kind and loving, Sayaka makes Celestia's life so much brighter. With Sayaka, Celestia can be herself without fear of being considered ordinary and uninteresting. This is literally the final push to create a perfect life; the puzzle comes together, everyone is happy and content (especially me)
