But on the sentence beginning

You may start a sentence with 'but', but you want to use it sparingly. The same applies to 'and'. It will often be used an interjection and be followed by a comma. In your example, 'happens' should be 'happened' to agree with the tense of 'understood'.

But he never understood why it happened.

But, he never understood why it happened.

This is for Carl's benefit, and I get an error message when trying to leave a comment.

Because this is a site for teaching and learning proper English grammar, your answers should use proper English. 'I' is always capitalized, always. Your sentence is a comma splice, and it should either end in a period after 'me' or use a semicolon to make a compound sentence.

It's OK for me, i think it's correct.

....Thank you so much !