It's annoying asf when people fucking say this shit. Family, strangers, and some females say this bullshit.
Most Helpful Girl
Maybe it's because you are0
Most Helpful Guy
Chances are it is partially due to puberty and teenager hormones. There is a good possibility that you are simply quiet. When people are quiet, people tend to assume they are serious.
The difficult thing is that people can only really see into your emotions based on expressions you give out. So even if you are estatic all the time, people won't know you are unless you let off a smile or crack a joke.
So my advice to you is to be more expressive if you can. Laugh, smile... It's these things that let others know "hey, this person is a fun guy."2