Extend - Wrong End Of Line

I'm running into this about half the time. I extend a line dynamically. I choose a point near the end I want to extend. Instead of extending from there, it shortens the other end, bringing it to where I clicked. So a simple two click extend takes several as I extend one end, then the other that was shortened up, then have to trim the other, or else snap to something during extending to get it to where it was before the extend took place, because that end was never supposed to move. I'm missing something obvious. I would *assume* that whatever side of the midpoint you pick that is the side that will extend dynamically. What is the trick?