We are genetically programmed to be motivated to seek food, love and sex, and numerous environmental influences can alter these genetically-based motivations.

The question I am having trouble answering is:

How genetically-based motivations can be impacted by environmental influences?

I have chosen one motivation to write about and that is love.
Some questions that will help answer my inquiry are:

What is the cultural definition of love?
How does this differ from your definition?
How are relationships defined by our culture? How is marriage portrayed?
Please cite sources.