---------------------------------------------------------------------------------------------------------------------------------

* Correction:     Student's sentence        Corrected sentence        Grammar explanation       Better sentence    

They say happiness is a choice. I think i  agree with that. How we react to circumstances determine our feeling, either we dwell in unhappiness or opt to move on. 

They say happiness is a choice.
This sentence is correct!

I think i  agree with that. 
I think I agree with that.
The pronoun "I" is always capitalized, no matter where it falls in a sentence. 

 How we react to circumstances determine our feeling, either we dwell in unhappiness or opt to move on. 
This sentence is correct!