I don’t understand market psychology

Can someone please explain to me why gains cause panic? I constantly see people asking “when do I sell this?” “How much higher can it go?” And all other kinds of questions that lead back to, I’ve made money so surely the trend has to reverse. I don’t understand that. Why do all this research and learn about a stock, buy it when it’s down, hold through the turn around and as soon as you go back above break even start saying “well surely I have to sell it now”. I understand the whole concept of overvaluation and blah blah blah but if you believe in a stock long term why do you just expect to endlessly chop up and down? And if that’s what you believe why buy it at all? Just doesn’t make sense to me.