For example in I'm afraid of the dog (not general, but at the moment, because it is growling)
According to the Collin's English Dictionary the phrase is using in two meanings.
Firstly, it is mean as 'feeling fear or apprehension'. So that can understand the first my explanation.
AFRAID is developing as regretful (something negative). As example I can put on this sentence: I'm afraid I lost my temper.
It is good to remember that AFRAID as part of speech is calling as ADJECTIVE.