Well... Because you are a human being with (occasionally anyway)
;-)
Thoughts, feelings, beliefs, desires, and opinions
That differ from other peoples.

Because people are fallible.
Its part of being a person.
People fail to take all things into account sometimes,
And I think it is a wonderful thing about being a
SOCIAL BEING
(which humans are as a matter of fact)
That we are well placed to
consider
think about
and possibly integrate
All of the considerations that people bring up
Whether or not they occured to one beforehand
(When the decision was made)

As for WHY one would dispute...
I'm aware my upbringing is fairly indiosyncratic...
But I was taught that saying what one really thinks...
Especially when one disagrees (even partially) with another...
Is a mark of
Courage (to assert ones own position)
And Respect (that another is able to handle a respectful difference of opinion)
and Faith (that different people have valuable things to contribute)