is it not annoying to be around someone who is always "joking"? (if are wanting to think seriously, be serious, etc)? why they are like that?

Sometimes it can be annoying. But consider this- if every person was exactly the same, the world would be boring, and also there would be no one to seek help from for the problems that we aren't suited to solve ourselves. 

If you can't appreciate the differences between different personality types, then at least learn to tolerate those differences. 

People like you who are serious need to go along to get along, to lighten up and let the humorous type people enjoy their silliness sometimes. And those humorous type people need to go along to get along, need to NOT refuse to be serious when a serious type person has something serious to discuss. Humorous types and serious types need to respect each other.

When humorous people are making jokes and laughing it's aggressive bull biscuit for the serious people insist that there's nothing funny, that humor and fun isn't allowed. And when serious people are trying to discuss something seriously it's aggressive bull biscuit for the humorous people to insist that there's nothing serious, to refuse to be serious. In both cases, it's dismissing and trivializing of those unlike oneself, insulting them as beneath ones consideration. 

Just choose people who are more serious and intense like cons and serial killers.

Could be they don't want to hear your whinny story, or get involved with your emotional distress.

There can be many different reasons.

They are a bully,   want to control the situation and conversation.

They could be shy or bad at conversation, joking around is their way of staying in the conversation. 

They don’t like being serious. I have one cousin like that.  He will even get physically funny, falling off a chair, just to turn the mood and conversation back to jovial, lighthearted mood. 

Or they don’t follow the conversation /topic at hand.  

If you really want to know do a little research in to it.

https://healthpsychologyconsultancy.wordpress.com/...

It can be seen as a sign of bullying, too. People who are always joking can be doing it as a subtle way of insulting and putting others down.

How to react,stay away from them.

I would much rather be around someone who is joking & playing around than with someone who is judgemental & constantly criticizing & putting me down.  You should feel lucky to have such a person in your life.  You can get serious when this person is not around.  Appreciate this individual for being positive & not negative.