responsibility and obligation

Responsibility is defined as a duty to deal with or take care of somebody
or something, so that you may be blamed if somthing goes wrong.

Obligation is defined as the state of being forced to do something because
it is your duty, or because of law; or something that you have to do because you have promised or because of law.

Mandatory is defined as something you must do as required by law

I used to think that we have a choice to choose and decide what is our obligation. However, it turns out that life is much more complicated. Sometimes, obligation is not optional, it is the same as responsibility and mandatory.