It's a phrase that implies a sense of questioning and perhaps a bit of uncertainty or even frustration. The person asking the question wants to understand the reasons behind the other person's desire for their love.
Here's a breakdown of the words:
* Por qué: Why
* tú: you
* necesitas: need
* mi: my
* amor: love
The sentence structure in Spanish is slightly different from English. "Por qué" comes at the beginning, and "tú" is used instead of "you" to emphasize the person being addressed.