The are a few people in your life who you consider closest and important. However, you realise after a while that it may not necessarily the same from the other end too when you don't get a reciprocation from the other end of a similar level to the effort you put in and went out of your way to express your fondness / care for that person.
Should you be disappointed when the above situation arises? You shouldn't be because if you did, you just made it a transaction and not a relation because relation / love should always be selfless and you shouldn't be expecting a return on investment( at least the ideal situation :p ).
And is it the fault of the other person? It most certainly would not be because you were never as special to the other person as he/she was to you as his/her priorities would be different!
But then, what's a friend if he is not a friend in need!
So, once you get to know this, you should reassess your priorities, identify your true friends and go about maintaining that friendship for life till you reach this point again and start all over!
