This can happen on both shorter time frames, like day trading and longer time frames whilst riding parabolic trends or swing trading. You may enter a trade with the expectations of say 10 or 12%, however when it gets there, because everything is going to plan, you start to shift the goal posts and feel like you can squeeze a little bit more out of the trade.
As humans we have a very hard time controlling our emotional decisions, especially when it comes to our finances. Even more so, we have a terrible ability to predict our future emotions or how we may feel or react when we reach out desired goal.
We often refer the cause of this as our own greed, and it may be true to a degree, but there is a little more to it on a deeper level which is known as Hedonic Adaptation.