Unfortunately there is no easy answer. Hair can thin from SO many different causes ranging from stress (yes it's a real cause of hair loss/thinning) to hormonal shifts, changing your birth control, thyroid issues, diet changes (lack of proper nutrients), improper hair care, toxic styling products, use of too hot water when wet-washing, aging, hereditary triggers, pregnancy, post-pregnancy shedding....and on and on.
The best thing to do is take a complete hair inventory which would include analyzing as clearly as possible when your hair started thinning/shedding. Try to remember when you first noticing the thinning. Was it a particular time of the year? Or had you just gone through a major life change? Did you change doctors, medications, birth control, thyroid supplements, vitamins, minerals...etc.
The more time you take to try and figure it out, the better chance you have of getting to the base cause and isolating the most likely reasons for your thinning hair.
Once you have narrowed down the possibilities, start to test your hypothesis by changing your hair related behaviors...whether that means changing your products, eating a different diet or similar.
Good luck to you.