From Merriam-Webster at http://learnersdictionary.com/definition/happen
HAPPEN verb [no object]
: to take place especially without being planned : occur
Mistakes/Accidents will happen.
Something like that was bound to happen sooner or later.
You never know what's going to happen when they get together.
[+] more examples
not used in progressive tenses
a : to do or be something by chance — followed by to + verb
The children happened to be asleep when we called.
I happened to overhear their conversation.
I happen to know his brother.
Do you happen to know the way to the station?
b — used with it to describe something that occurs by chance
It (so) happened that the children were asleep when we called.
It happened to rain that day.
As it happened, it rained that day.
It (just so) happens that I know his brother.
c — used to make an angry or forceful statement — followed by to + verb
The woman you've been looking at happens to be my wife!
I happen to think that the movie was actually very good.
"Your father defends the right-wing of the Republican party, and I happen to think you almost got to be - like a demented lunatic," is meant as hyperbole by the character who is speaking.