No one in the world can take place of other one

When i was kid, i heard a lot about Sunil Gawskar's attitude and nature. He was a great person, in fact he is a great person ever. There has been an incident when one reporter has told Sunil that Sachin had takeover his position and he was being performed very well also. So Sunil have smiled and replied that "no one in the world can take place of other one". He was right at his place. When someone comes in place of other person, He makes his own status, his own identity or own importance. The place of previous person can't be affected.

But Time washes all mysteries. People were afraid of dinosaurs. Time passes, now they are in books only. Many beautiful birds and dangerous animals are not present right now. People, very rarely read about all these and then talk once and then forgets about them. All respects being washed with time. Recently i have visited Rajkot. Some statues are there e.g. Princess Zaansi, Indira Gandhi, Swami Vivekanand. None of them are being renovated or taken care of. People just reminds name of them, and the spirit spreads down on the earth.

I was talking about this topic with one of my friend. He laughed and said in case of relationship, once you come across someone, no one else can come in your life to take that place. And just after 5-6 minutes, he has started blaming some other friends. And i shocked that just few days back he was praising the same guy. Few days back situation was like he could not even live without the same person. He were used to tell everything and share each and every moment together. And now as per his point, the same person is not being able to understand him. The person is not giving him enough time. But as much i know that person still have the same feelings for him. As i know both of them very well. Now a days, he is used to share everything with some other friend who is understanding him very well (as per his point of view).

If no one can takeover other's place. Then how one person who is same, have not been changed a bit even, have been less understandable. How do the same person been liable to blame everything while nothing has been change between them.

Is it matter of time or matter of needs or nature ? If no one can take place of other one then why after marriage spouse starts crying on each other's shoulder instead of parents. Why after getting child a woman start satisfying her child instead of husband. Many people have more then one lover, Why don't they even think about older.

I am not getting any answer about all these. If life is all about keep changing then why do people keep promising. Why don't they look back and confirm that he has not broke any heart or he has not left any important place which can't be filled again by anyone else.