As a modernist poet, E. E. Cummings prioritized self-expression, which included a celebration of individuality, emotional complexity, and the subjectivity of truth. The 'force' of his poetry can thus be measured (although Cummings, I'm sure, would object to that) according to those characteristics.
The first stanza of this poem offers an explanation of sorts as to why Cummings tended not to use capitalization or much punctuation in his poetry:
"since feeling is first
who pays any attention
to the syntax of things
will never wholly kiss you"
Syntax refers to the grammatical arrangement of words to create sentences, or, more broadly, to the grammatical constructions which act like moulds with which we shape our language. As a proponent of self-expression, Cummings believed that to shape our language, a tool of our self-expression, according to these hard and fast moulds would be to curtail the potential of our self-expression. It would be like pouring water into a glass when you could let the water run its own course.
The last line of the first stanza, "will never kiss you," suggests that one who curtails his or her self-expression according to a pre-defined syntax, will never be a truly passionate person, or a person capable of love in its most intense, personal form. The poet insists that "feeling is first" and that feeling, therefore, should be free to find its own way outside of any given syntax. This is the main reason why Cummings, in "since feeling is first" and in many of his other poems, refuses to use capitalization and correct (in a prescriptive sense) punctuation. To do so would be to inhibit or hinder (rather than help) his own self-expression.