Some guys and girls change easily. There feelings might be effected by bad news,friends,something they saw or heard, etc. A lot of times they change because their growing and experiencing a new form of life. But some don't change at all! But it sounds like the person your talking about got bad news,or heard things he shouldn't of. This person may of even got bad news from his friends! Anyway Hope this helps :-)
As people get older they normally gain more control over their emotions and their behavior is driven more by what is reasonable than how they feel. There are exceptions but generally, guys start out like what you describe and by the time they get to grandpa's age they hardly change at all from day to day. Some even become resistant to any change at all.