Really good book.
The key idea is to be happy, you frame any and every experience as a gain. More specifically, the key concept thoughout the book is that you must stop living the GAP and start seeing the GAINS.
As the following figure (also featured in the book) shows, if you measure your progress, experience or event by comparing something to ideal state, then you can always stay frustrated with guilt and disappointment. However, measuring this conversly through looking backwards, we always get positive measure and thus this shows that you actually have gained, meaning your confidence, satisfaction increase and you can succeed in more deliberate and self-determined way. You will be happy regardless.
In particular, authors do not tell that having ideals is wrong, in fact is rather necessary to make any progress. Thomas Jefferson’s ‘Life, Libery and the pursuit of Happiness’ is that happiness should aspired to, though unattainable. The book very clearly outlines that happiness can be attained, through GAIN thinking instead of prevailing GAP thinking.
The thinking should be accompanied with self-determined success criteria, not external, because then you are in control of your GAINs. This includes taking ownership of your experiences, because who controls past, controls future and who controls present, controls past. GAIN thinking should be practised daily and thoroughly, because then you exponentially start to notice GAINs, stay happier and all other possitive effects kick in. On top of that, in this way, you dictate your days if you reflect and define GAINs for your passing and coming days.