Okay, if you're really daring, then you're willing to brave the Prager Challenge, a mountain of reflection.
Prager cites research that suggests that young adults are still developing their identity. Specifically, she says:
"Becoming part of a couple and being recognized as such by others can also shape young adult identity (Troll, 1985)...young adults may identify vicariously with their partners' attributes or accomplishments (Beach, Mendolia & Tesser, 1992) or with one another's affiliations...Becoming a couple also carries with it the risk of losing certain parts of one's identity, however [e.g. the process of 'dwindling' for some women]" (Prager, 1995, p. 132).
So, this question involves some synthesis of concepts already discussed and your own gut's wisdom gleaned from observing others and your own relationships. Why is becoming "fused" with another person (or adopting some aspects of their personality / interests / feelings) more challenging in young adulthood than it might be in middle or late adulthood? In other words, what is the attraction of losing part of your personality even if that personality is still evolving and emerging? Give it your best guess.