According to this article lowercase are friendly :
Words set in all lowercase letters are informal and tend to feel
friendly. Variations in form, rounded shapes, and no initial capital
letter give lowercase words a common, conversational feeling.
and uppercase are demanding :
Words set in all caps feel important, powerful, reliable, and
enduring. The letters are big and demanding.
Additionally, according to this article lower case are much easier to read :
At body text sizes, capital letters—or simply caps—are harder to
read than normal lowercase text. Why? We read more lowercase
text, so as a matter of habit, lowercase is more familiar and
thus more legible. Furthermore, cognitive research has
suggested that the shapes of lowercase letters—some tall (dhkl),
some short (aens), some descending (gypq)—create a varied visual
contour that helps our brain recognize words. Capitalization
homogenizes these shapes, leaving a rectangular contour.
Just adding some extra reference from this article :
Friendly or Unfriendly?
Friendly visual features could be described as positive, while
unfriendly visual features could be described as negative.
Friendliness is not only determined by what is said, but also by how
it is said (i.e. the tone of the conversation). Our tendency to assign
and characterize personality based on conversation is easily
recognizable in the example below. This example uses contrast, visual
weight, , color value, size and typography to alter the meaning that
is conveyed by the words. The content conveys the message, but the
look and feel change how that message is interpreted, altering the
Which of the statements below would you rather hold a conversation
with? Which one do you feel more compelled to approach or avoid? Which
one naturally grabs more of your attention? When it comes to
conversation, someone has to lead, and opposites attract.