Why does tension act in opposite directions on two connected masses being pulled?

[tica86]

This is a general question about tension.

Two mass m1 (10kg) and m2 (15kg) are pulled with a pulling force of 50N.

It was explained to me that the tension in mass 1 acts to the right and the tension in mass 2 acts to the left, why??
If they are being pulled in the same direction(to the right) then why isn't the tension acting to the right for both masses??

 

[The legend]

how are the blocks placed? and how is the string(rope etc) connected?