why  (a+b)²=a²+2ab+b² ?

Why (a+b)²=a²+2ab+b² , Have you ever thought ?

  Its more than 25 years when I studied  (a+b)²=a²+2ab+b² , and then studied on several situation e.g. in my bachelors degree study, in my master degree study. But never ever thought other than to just remember :  (a+b)²=a²+2ab+b² . So I just mugged up this in my mind, recently on some situation I tried to realize…