For this case I will give my niece as an example.
During her age between 15 and 18 years, she has had important emotional changes.
Among these emotional changes are:
1) Happiness: She gets quite happy when she goes out with her friends or with her boyfriend
2) Sadness: When she argues for some reason with her boyfriend, or with her friends, the day is a little sad and this may remain so for an indefinite time.
3) Resentment: She has a grudge for people who have tried to hurt her.
These emotional changes can be suffered by anyone, but are more critical during adolescence and requires the support of parents to know how to manage.