Ah! So I'm not the only one who has this experience (!)
I just left a message on his blog: maybe it's a "guy thing" but I think we suddenly get competitive when there are other people around, esp. when doing the same thing.
Most of us are brought up to (constantly) compare ourselves with others (I think that is one of the first topics I learnt in my social psychology class that many of us build our "self" and sense of existence based on how other people perceive us etc) ... but as we grow older, and gain more confidence in ourselves, we begin to relax more. These days I just show up to my yoga classes with a block; the message is clear: yes, I need help and, yes, I need to modify cos there are asanas that I simply cannot physically do and achieve but it's okay, the point is not to do the perfect pose but how I can still benefit from a pose with modifications. How other students/yogis do their poses/practice is really their business; but it is also okay to admire their amazing poses because yoga is such a beautiful art form too! It's also okay to be inspired.