Mine comes from the fact that my name is often pronounced as in Italian or Spanish : "Paulina".
So, to make it short, I thought "Lina" was well ( I hate being nicknamed "Polly" or, even worse, "Paupau" !
) and other people start calling me this way.
After that, I learned "Lina" means "soft" in Arab ( thanks Asma !
), which is not quite my temper, but is still nice anyway !