It seams Ashtanga shelters 2 ambiguous motivational and work attitude concepts:
1. The first one is: It gives possibility to work putting out small motivational aims. The practitioner feels happy achieving them, and seeing his progress.
2. The second work attitude is: To provide a well balanced daily exercise program which provides for long term and good adjustment of workout load.
This two might seem contradictory. Let me explain: The first concentrates on short term achievements and on achievements in general. The second tries to maintain long term balanced practice, so that one could practice “when 100 years old”, or practice for the aim of own happiness, enjoinment. It some sort of Achilles dilemma, as in Iliad his mother has prophesied him “either he goes to war achieves great fame but dies, OR he lives happily for 100 years rich and with his children, but no one remembers him after.” We know the Achilles choice … But let go back to Ashtanga. The first way of working is greatly motivational and allows great achievements. But it is risky and it is deemed. Some day one reaches limits of his body limits (in time as regeneration capacity, adaptation speed, or just plain physical joint-ligament anatomy peculiarities). So, what comes next? How with next motivational heroic deeds? In trying to acheave more, one ends eventually in injuring himself.
On the second path, one lives happily with his daily careful, cautious practice within his limits. No great achievements, no great injuries… No motivational “Hurrah! I’ve got my heels in Kapotasana!” or “I’ve got my wrists in Pashasana!” Ye, you might say but seems pretty healthy. The problems are, first where to get the motivational impulse and second how to get explore and expand his/hers own limits. In short boredom and self complacency is the very probable premature end of this way of practicing. (Sorry but probably no 100 years anyway, except one finds other motivational).
Some additional related thoughts: Because of this contained ambiguous attitudes Ashtanga seems well appealing to achievement oriented types, well motivating for daily routine, but also well adjustable for curative and rehabilitation practice. The last is as it allows prescribing an adjusted set of exercises, modified according to the aims of rehabilitation. The well documented routine allows also measuring well the progress of the curative measures. It might sound counter intuitive as it contradicts the first tendency of “faster, longer, deeper”, but if the modified exercise set is executed with determination and discipline within the prescribed limits, the chance of re-)injury is nullified.