Friends

I came to realise, that for friends to stay as friends forever, it is best not to add an additional relationship to it.

Dont start a business with them.
Dont recommend them to work in your company.
Dont work on projects with them. School projects in my case.
Dont agree to rent a house with him/her.
Dont lend them money.

And why is it I know the concept of this, but I still do it all the time.