Why hurt the heart?

Why do we keep hurting the people we proclaim to love and loves us?

This is question I want to find the answer the soonest time possible. 
It is very difficult to reconcile how love and anger can be compatible. 
This is explanable yes, arguably understandable but in thoughts, matters and beatings of our heart or in neurology, the parietal lobe, it is still unbearable. 

I'd like to know why this world though how much love we paint and understanding we deliver, we are still bound to commit one of the biggest and most fatal error - that is to hurt the people we loved the most. 

I don't want to sound so naive nor experienced but it can be observed that most of the time we are culprits of the most beautiful and the most hurtful love we can give to one another. I'm thinking whether that's just the normal cyclical dynamics of our existence or do we really have the best hand to rule over these circumstances. 

I have been in loved not long ago. It was not a perfect love but I know I gave ample amount of love and faith into it, but then in a lot of occasions I fell short, committed a mistake and hurt the person. This is sad definitely, I feel bad and I try to undo things by becoming better, but in another moment, I may have hurt the person all over again and yes, I felt sorry. However, I try to figure out now how is this related to our existence now? Is it really a part and parcel of our being or maybe we can shortcut the situation and proceed to loving? 

I still do not know the answer. All I know is that I am in the process of discovering how it should, would and could have been... a little bit less painful. 

I am sorry for all the heartaches I have brought and believe me, I am miserably sorry.