difference in value of same coefficient.

hi :)
i started to learn simple and multiple regression, and there is one thing i can't understand.
i used data frame with values of hindrance, inhibition, and negative effect.
when i predict the value of negative effect by simple regression, using inhibition alone, i get its coefficient.
but if i try multiple regression - predicting negative by both inhibition and hindrance, the coefficient of inhibition suddenly changed.
(from-> -10 to -> -12)
i thought the coefficient shows the change it cause in the dependent variable.
if i still focus on the same independent variable (inhibition) why did it changed?

thanks a lot!! :)