Why are humans affected by mood?

Only quite recently and for most of the time in uni, did I start asking myself this question.

Why are my mood and emotions so easily affected? How did i last through the past few years? I don't remember that i was facing such a problem in the past? If i ever did, it was due to quite a specific problem that i can pin point and it doesn't last for long.

However, this has been going on and off for the duration of uni already. Is it study stress? Is it the need to always fill up my time with some activities? I can't get used to the life during the holidays where i don't really have to meet deadlines or chiong some studies?

Well its not true that i'm not filling up my time with activities. I did have plenty of activities, and going out with lots of peps. However, it just didn't quite feel enough. Whenever a day ends, i feel quite sad as it is time to transit to the next day. However, nightfall is also when almost everyone is home, and almost everyone is free.

I do not like my current lifestyle, and i do wish to change towards a lifestyle which is less reliant on others. Maybe i will have to start getting used to doing more things alone and yet achieve a high efficiency. After all, everything is done for my own benefit.