everyone will surely regret it if they have gone through times of altcoin bulls reaching new ATH and has been followed by a correction. because the new ATH is unlikely to happen in the near future, but it takes a long time. and if we can sell altcoins on new ATH then we have a chance to get bigger profits. and we can buy it back when the correction occurs.
Taking profit during bullrun is good because you can enjoy your earnings. We'll never know when might the price decrease again after reaching the ATH thus to avoid having regrets dont refrain yourself from selling if thats what you think the best for yourself. But there are investors who are firm to not sell even the price reach the ATH then turns downward again These people are determine to keep holding until the price reach their desired value to finally sell.