I think it will grow higher aswell but since the price literally skyrocketed the last few days its hard to believe that this trend goes on forever up to 8000$ without a hard drop before. And once it goes down alot of people are going to panic-sell aswell because they just see the decrease in value and that's the point where im buying again.

Also people sell at any price as long as there is a demand for it. Maybe they need the money or they just did a one time investment into bitcoin and want to be done with it because they think its to risky. There can be lots of reasons. But you're right, if you're in for the long term then you shouldn't sell right now even if it goes down again.