Some people just want to please others. They will do most anything to be accommodating . It is just their nature. They derive sincere pleasure from being amenable and giving. They go out of their way to do for others all that they can. Very often no unkind words come from them. I think they subscribe to the belief that they are on the planet to make a difference in the lives of others.
Then, I do think there are those who are too nice and too passive simply because they believe it will make others like them. They agree with what I say when it is just the two of us and then when someone else is around they agree with what they say even if it is the opposite of what they just agreed to a few minutes earlier. I have known a few people like this.
It is not for me to judge however why someone is too nice or too passive I of course am not in their head so do not actually know why they do it.