Mate, in case you aren't stirring I'll give you a sensible answer.

However sure you are that a horse is going to win there is still a chance that it won't. Something can always go wrong, from paid off jockeys at the sceptical end of town to faulty gear, to a very attractive filly in the next stall putting our boy off or maybe the other jocks will box our short priced favorite in, I've had that happen to a dead cert. There is always a chance it will not win.
Let's say it's a very good chance and that in 9 out of 10 races nothing will go wrong and it will win, that still leaves a 10% chance it won't.
If you can get $2 about this horse you're laughing. 9 times out of 10 you will come out in front (and end up with $18 from every $10 you bet).
If you can only get 1.05 about it cause everyone else knows it's going to win too YOU SHOULDN'T BET ON IT. Maybe this time you will win and make a little cash but if you bet on the same sort of horse 10 times chances are one of those 10 times you will find something goes wrong (10% chance of it). So for every 10 dollars you bet you only get $9.45 back.

This is a case at the very short end of town but the same applies all the way out to the longshots. If the price you get isn't worth the risk of the horse not winning then don't back it. If you do back these underpriced horses consistently you will come out behind.

Hope that helps,

KV