If a football team loses the first game of the season, that doesn't make them a horrible team that can't find success later in the season. The same applies to our health - one meal of poor nutritional value doesn't mean we can't live a healthy lifestyle.
Keeping a healthy perspective about future outcomes and learning from our failures are key pieces to mental health as we move through life. Having an extreme reaction to a loss/failure could prevent us from realizing future successes.