Each of the sections below has a single answer, and the five sections are related. After finding the solution to one section, the others should be much easier.
The Best Friend
Whenever you're down and out, I'm in there like a dirty shirt.
Your other friends and I are gems.
When you renovated your bathroom, you needed me in order to work hard.
If you're angry, I will come and provide some help.
If you're still angry, I will let you be, to help you evaluate.
I'll be with you in all your despair and anguish.
If you're waiting on too many things, you can bring me in to shake things up.
If anyone is being ruthless or unforgiving, I'm in!
It is probably wise to remove me from employment.
The Party Animal
When I get together with any girl, there's bound to be a party.
In fact, no adequate party happens without me being there.
I cause a commotion at any party just by showing up.
The Efficiency Expert
Any young man can make use of me to get ahead.
Stick me on a letter, and it will arrive much faster.
If you were thinking of leaving your job and you brought me in to consult, I would advise not saying anything.
To what does each of these sections refer? What is the common bond between them all?