Life In 19x19

Is there a rule of thumb when one should attach to an opponent's stones? I know it's supposed to make both players stronger when you attach, but besides that I'm not too sure.

There are different reasons for good attachment:
- forcing a preferred direction
- mutual fair settling
- defense of weak stones by attaching already strong opponent's stones
- if efficiency is greater than in the case of non-attachment
- test move
- only successful tactical solution to given aims
etc.

A good rule at your level is to only attach when you are trying to save your stones and need to make shape quickly. Never attach when attacking unless you can read out that it achieves what you want.

Of course, as you get stronger you will discover that there are exceptions to this rule, as there are with any rule. And I would recommend breaking the rule just so that you see what happens.