What do you think mercutio thinks of fate or destiny? why?

 Mercutio is a young gun and having fun. He doesn't take anything too seriously, even women. He is young and reckless for the most of it, yet like all young men, there is a budding man underneath. 

Mercutio will not tolerate injustice, is very loyal to his friends and ultimately he delivers "a plague on both your houses" - this pivotal delivery wouldn't come from a fellow who thought that fate and destiny are a lot of cobs wobble or outside of his control. 

He is a young gun and having fun, though he is also a pretty layered character. I think he does believe in fate and destiny to a degree, though like most of us, doesn't spend too much time placing his fate in the hands of the gods, he creates his destiny by creating his destiny - until his fate is taken outside of his control.

Because writing on paper is said to engrave what your writing down into your brain therefore writing your thoughts is better than speaking them.