It is very sensitive and touchy question. Each and every person of the world loves their parents because we have strong blood relation with them. After God they are the most respectable and loveable persons on earth. Our love for them is very natural thing; there is not any force which is compelling us to love our parents. Even a child who has not seen his/her parents from their birth to younger age, they still love their parents.
We love them because they do so many works for us. Try their best to give all the facilities of the world to their children. This love is pure from both sides either from children side or from parent's side.
There is not any solid reason or proof, that we can say that, we love our parents for this reason. It's just a natural feeling. This love remains in our heart until our death.
Our parents are the one, who understand us and fulfill our all needs, before we say something. And as a result they just want love and care from their children, which is their right. So we love them and care for them as much as we can. and also very possessive about them.