For Mental strength 

You would have spoke to your girl/boy friend every day, every minute and yeah updating every single thing you have done. They say nothing is permanent in this world. So once you experience break up there would be no communication between you and your partner. You might feel you are lost and you couldn't bare his/her absence as you got so much habituated to them. Well, this is the phase which you become mentally strong. You will understand that life is normal even without his/her presence. This truth realization will help you through out your life.