Expectations in a Relatrionship

I am confused !
Relationships have always confused me !
I realise this mainly during the troubled times!
It takes time to define a relationship, its nature and objective. Only time can tell about the longevity of a relationship and if it is successful or not and if it is ongoing or not!
And then, there are a few which fall on neither sides in both categories. We don't know sometimes if a relationship is alive and kicking or if it has ended successfully or not!

Humans are amazing - not only because each one is unique, but also each heart and mind store infinite feelings and thoughts about people and relationships with them.
We get hurt when our feelings are not acknowledged or in other words, when our expectations are not met! Feelings arise from the heart. From where do expectations arise?
And a more critical question is - Do we have control on our expectations?If yes, to what extent?

Can spirituality answer these questions? Many religions and philosophies seem to be against expectations in a relationship!
The core principle of Buddhism is to have no expectations which is also the basis for almost every other religion as well. BhagvadGita dictates one to do the duties without worrying/expecting the results!
But, is it easily humane to be without expectations in one's deeds or in a relationship?
And my bigger question is : If there is no expectation, what defines a relationship and how does one differentiate a relationship from another?