Thanks everyone for your comments.

Let me make it clear: I do not judge people by their looks. If someone appears off my standards and looks happy, I am happy for them; if they appear miserable, it makes me feel sad, although I do realise the link I may make between the various aspects of their looks and their state of mind is subjective, and as such, prone to error.

My problem is, indeed, that regardless of how much thinking before speaking is in my best interest, I simply get carried away - even if I do recall the committment at the beginning of the conversation, I get lost half way through it.
This happens even if I have to do with someone who has repeatedly abused me verbally, isolated me from most of the open-space team and sabotaged my career on a daily basis. Even then I tend to say things which may get such individuals irritated.

As you can imagine, compared to such situations, not hurting someone's feelings of someone nice and polite with a comment that comes across as criticism of their looks is less of a priority, even if the experience of the outcome of hurting someone or getting hur is rather similar: shame and disappointment.