so, what could be the possible reason for a human to like to suffer through sitting for a portrait rather than just taking a great selfie?

well, there could be many reasons all tailored to that human's personality type:

a. they could be just your run of-a-mill art nutter, loving to see what kind of colors, lines and form their image inspires

b. they could be a person who notices that the photograph will not reflect their likeness as deeply as the painting will

c. they could be someone who loves to participate in the process, have a say in many parts of the painting, starting with selecting the pose down to discussing overall color scheme

d. they could be a do-gooder type who'd like to donate their image, contribute to the work, etc. etc.

e. they could be an investor type - who wants to order a painting from an artist and be able to have an object of great value once the artist croaks

all of the above reasons are good enough to pick up the brush and start putting the paint on my proverbial canvas