I believe this is a quote from John Maxwell, and I love how he speaks about attitude, and so many other things in life. It is quite interesting, if you see John Maxwell, he doesn't seem like this guy carrying his nose high, but there is still incredible wisdom coming out of his mouth and from his pen in all of his writings.
A positive attitude is not a destination, it's a journey. It's not a one-time decision, but a series of choices every day.
It is incredibly important to make the right decisions, not only once, but day by day. If you want to change your life, it isn't enough to decide to stop smoking one day, but you need to keep that decision in hand and live it every single day from the day you decided to stop.
Source: Bing Designer
It isn't enough to decide to live a healthy lifestyle once, but you need to do it daily.
And the same is true with your attitude. It is so easy to fall into the trap of a negative or a bad attitude, and then you will fall back to where you came from. You need to decide daily to live healthy and to have a positive attitude.