I think that you are partly right when it comes to unrequited love, but I think that it is hard to love without fantasizing, and fantasizing generates expectation in many cases. Also, love can be painful without expectation: for example, I had to be apart from my love for a year just after we got married, and it was very painful, but not because of any expectation.
I think love can be painful for so many reasons, just as it can be wonderful for so many! How about when you love someone deeply but they are making mistakes with their life and going down a self-destructive path? That is possibly one of the most painful things you can see.