I think it's because Itsuki was born and raised in the kind of environment that nurtures him into his best, have courage and explore his passion, as well as his strongest suits.
Meanwhile 2nd ML (what's his name again? It's past midnight over here ugh) was born and raised in a rather traditional Japanese family, hasn't got the time + chance + resources to find out his, err, 'ikigai.'
Plus, boys mature slower than girls, and I guess 2nd ML is quite close in age to Yuki.
Plus, Yuki is not any ordinary girl. In this sense I mean there're complications involved dating/marrying an audibly impaired person - it takes great deal of patience, perseverance, etc. and overcoming social/family's pressures.
Thus, his lack of confidence.
poor boy, the thing about most 2nd MLs is they wait too long or never have the courage to do anything so when ML swoops in its free real estate