Nothing Tastes As Sweet As What You Can't Have

Nothing tastes as sweet
As what I can't have
Like you and the way that you're twisting your hair
Round your finger
Tonight I'm not afraid to tell you
What I feel about you.
I'm gonna muster every ounce of confidence I have.

These are some of the lyrics to Teddy Gieger's song For You I Will. I really believe there is an awful amount of truth from these words. In a play I'm currently in, Singing In The Rain, one of the lines between the main character, Don, and his closest friend, Cosmo, goes something like the following...(pay attention to the bold)

Cosmo: Three weeks are you are still thinking about her?
Don: Yeah she is on my mind
Cosmo: How could you not, she was the first girl to not fall for your lines since you were four...

Why does this have to be true? Why does our heart ALWAYS lead us to someone who will never like us, or at least don't at the moment. Why is it that those we always think about are those we can never have? Sometimes this can be very upsetting...