How can the difference between "comfortable with doing sth" and "comfortbale doing sth" be best explained?

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I thought about it long and hard and I really dont think there is a major difference between the meaning of the two.

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After 'with' the -ing form is called a gerund and describes an activity. In other words the verb has the character of a noun.' I'm not comfortable with speaking in public' 
.Without 'with' the -ing form can either be a) a gerund  as it's not necessary to use 'with' ' I'm not comfortable speaking in public'
or b) a present participle describing an incomplete action.' 'I am comfortable sitting here'

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