This is the 'undefinable' aspect of gardening - it is a form of expression; a way of creating. Apart from following general principles of what goes where; and how - the rest is pure individuality.
Gardening is not a purely physical activity, it also affects one's sense of self and position in the overall view of one's environment. Time spent in the garden can often change one's view of life in general, too. Usually this results in a better outlook on life. Let us look at this, for a time.
For starters, you are outside; and we will presume it's a nice day.
Surrounded by colour, sights and sounds of nature, the aroma of fresh turned soil, the individual scents of the plants, breeze on your skin. Fresh air - can't do without it; can't get enough of it - no point in going any further into the benefits of air. Excercise - once again, something we cannot do without, for too long. These things can be acquired through innumerable activities, gardening, however, offers something special - 'me time'.
Gardening, overall, can be seen as a series of repetitious tasks, but whilst being involved with this activity, it never seems so (Unless of course, you really don't like it). There is a sort of meditative process that seems to instil itself into the gardener. Time for reflection, contemplation, maybe working through the days tribulations.
Ostensibly working, and you are, gardening though is not a violent process and you can take as much or as little time as you wish. This cannot be forced, either, plants will only grow at natures pace, therefore, whether you like it or not - you must learn patience.
There is some private excitement involved, also. Each time you enter the garden, there is the anticipation of - 'what is new today?', 'Is that plant's flower to open today?' Anticipation also for the coming season, too.
This creative tool, that is your garden, allows you to express yourself. Gardening is therapeutic, physically, mentally and spiritually.