The construction of ´I need/ You need etc´ is constructed with
personal pronoun in dative + lazým.
So it HAS to be
Bana lazým
Sana lazým
Ona lazým
It cannot be ´benim için´ if you want to express that someone needs something.
Or, when you use -vermek-
Bana bir bardak çay verir misin? - Can you give me a glass of tea?
You cannot say ´Benim için bir bardak çay verir misin?´, it´s weird.
But you could say
´Benim için de bir bardak koysana masaya´ - Put a glass ´FOR ME´ on the table as well (in other words, the glass will be meant for me to drink from, and the person it is said to, probably forgot to put a glass for ´me´ or smt) I hope I could explain 
If you say:
Senin için geldim or Sana geldim, they have to different meanings. The first means I came for you (I came to see you, I came because I know you would like etc), the second one means ´I came to you(r house).
In some cases, as you see, they are not interchangeable (gnerally depends on grammar rules), but sometimes they are, depending on what we are tyring to say.
Sana çorba yaptým - I made you soup
Senin için çorba yaptým - I made soup for you
Sana hediye aldým - I bought you a gift
Senin için bir hediye aldým - I bought a gift for you
Sana or Bana or any of those, have more the side-meaning of ´in the direction of, towards), Benim için, senin için, has the side-meaning of ´intention´.
I hope this made it a bit clearer and any grammar mistake sin my Turkish sentences please be corrected by natives.
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