self interest & work as a spy

Law 13

When Asking for Help, Appeal to People’s Self-Interest,

Never to their Mercy or Gratitude

If you need to turn to an ally for help, do not bother to remind him of your past assistance and good deeds. He will find a way to ignore you. Instead, uncover something in your request, or in your alliance with him, that will benefit him, and emphasize it out of all proportion. He will respond enthusiastically when he sees something to be gained for himself.

This is definitely one to remember. People always want to gain something out of everything they do..even if its for other people. If you show them where and how something will benefit them, it makes sense that they will come to you and want to help you even if just to get something for themselves. I think this is very realistic. People do it everyday... They may not be looking to gain anything they can touch but appreciation or recognition is something desired too.
The 48 Laws of Power by Robert Greene
Law 14

Pose as a Friend, Work as a Spy

Knowing about your rival is critical. Use spies to gather valuable information that will keep you a step ahead. Better still: Play the spy yourself. In polite social encounters, learn to probe. Ask indirect questions to get people to reveal their weaknesses and intentions. There is no occasion that is not an opportunity for artful spying.

I think everyone does this on some level. They may not do it in order to use it against the person at some point in time but we all gather information for a rainy day. You never known when you may need it.