You are everything to me.
sei TUT-to per me — stress on 'tut-'. 'Tutto' has two syllables: TUT-to.
A total declaration of emotional importance — this person is everything. Said in deep, established love.
'Sei' = you are. 'Tutto' = everything. 'Per me' = to me / for me. 'Tutto' is masculine singular even when addressing a woman — it refers to the whole of experience, not the person's gender. Simple, complete, and absolute.
Non ho bisogno di niente altro quando sei con me.
I need nothing else when you're with me.
Completeness — their presence is sufficient for all needs
La mia vita ha senso grazie a te.
My life makes sense because of you.
Existential — they give meaning to existence
Senza di te mi mancherebbe qualcosa di fondamentale.
Without you I would be missing something fundamental.
Hypothetical absence — shows what they provide cannot be replaced
The expression 'sei tutto per me' is direct and absolute — Italians do not shy away from total declarations. However, the partner's response acknowledging it as a 'responsibility' reflects Italian emotional wisdom — being someone's 'everything' is both beautiful and weighty, and should be handled with care.