Question

why atoms combined with each other ?

Answer 1

to form molecules they combine

Answer 2

Hi,

Here is your answer

Atoms are very unstable due to their unstable electronic configuration. They try to become stable like atoms of neon, argon etc. They want to attain octet electronic configuration (8 electrons in outermost shell) and in some cases duplet electronic configuration (2 electrons in the outermost shell). 

In order to attain this stability, atoms combine with each other.

I hope I have cleared your doubt.