Sofia was the love of César's life because
a) He hadn't had the opportunity to have sex with her yet.
b) Pelayo's interest in her made her more appealing.
César never got the opportunity to screw her over like he did Nuria which is how he probably treated all of his other past conquests. The accident interfered with his usual games and since he relied solely on being beautiful he was unable to properly pursue her which automatically turned Sofia into his what if and the possible love of his life. He seemed fixated, not in love.