The first time I had a dirty carb.
The second time my spark plug cap crapped out.
The third time was because I forgot to turn the petcock to "on". (This has been repeated multiple times.)
The fourth time I needed to turn my petcock to "reserve." (Has also been repeated multiple times.)
The fifth time I had run out of gas.
The sixth time I had fouled a spark plug.
The seventh time my spark plug cap had come a little bit loose from the wire.
I'm probably missing some, but that's most of them. Read Fred's Guide and ask any other repair-related questions in the repair forum.