How can I write a topic sentence for Shakespeare's Romeo and Juliet about love or infatuation, or perhaps about how teenagers are confused about love?

Certainly, you could claim that teenagers have a tendency to lack perspective due to their often limited life experience, and this tendency causes Romeo and Juliet to believe that their love is the only thing that makes life worth living. 

You could also claim that it can be difficult for anyone, but especially teenagers (again, because they lack life experience), to differentiate between love and infatuation, and this inability to tell the difference causes Romeo and Juliet to act impulsively in an attempt to honor their strong feelings for one another.

Further, you might claim that the heightened emotions surrounding the initial meeting of Romeo and Juliet, emotions that arise as a result of the long-standing feud between their families, confuse them and make them believe that they desperately love each other when it is really just the excitement of engaging in a relationship of which they know their parents would disapprove.