First off, success is different for different people. However the grass is greener on the other side. Now that I have put penned down cliche bullshit let me explain why.
Failure is the occurrence where the goal you set out to achieve could not be achieved. This is often due to changing circumstances more than laziness (at least from observations and readings). We all expect a linear world but we live in chaos and try to make sense of it through linear models.
There are times such opportunities Pass us by because someone else took the opportunity. In linear models, they are mostly zero sum games where the equation needs to balance. But in an infinite game, as long as you keep playing you will have a chance to win.
In the professional setting, it could be failure to get promoted. The promotion you traded your life, health and personal life for goes to a new person who just joined the company for a week. It happens (ok this is an exaggeration but since when do we ever think that the person who promotes above us is worthy, but situation deems them so). So the options now are to weep and cry, drown in beer and then head back to the grind waiting for another opportunity to arise, find new work, start your own business… Hey, suddenly the multiverse of possibilities open up when the original goal has died. The holy timeline you have been sacredly maintaining and pruning no longer seems that important (for context of future readers, Loki is an awesome show at the point of writing). And because…