Pretend friends. Ask me to go out and do this or that, but when it comes to going they always have a excuse.?

So I confront them on this, and now they won't even speak to me. I confronted her on what I believed was her affair with someone at work, and she was just using me to better her job. Told her to get help from her married man friend. Not that I'm really out anything, but why do people act with such dishonesty? In a way I feel used.