Lincoln's Melancholy

[ELCore]

What I also noticed is that many people described him as social and entertaining and how his expression would become animated when talking.
Could it be that - due to the stark differences in his facial features - people would notice the difference between happy and sad as far more intense that it actually was?

I believe that is Dr. John Sotos' idea in The Physical Lincoln. That Lincoln (to put it in my own words in modern terms) had "Resting Sad Face". (Confer.)