Only when it takes over.
If you bet a buck, and lose it ... well, big deal.
But if you bet your house, it's a sin because it leaves you and your family without roof over your heads. So if you gamble so hard you might lose something you or others can't do without, then it's a sin.

Also, if the gambling has power over you, it's a sin. If you can't walk past a casino or a poker table without a round, if you cant pass a lottery stand without buying in, then it's controlling you more than you control it - and therefore it's a sin.