When prepositions govern personal pronouns, as in stick to it, go with him, proud of it, etc., the primary stress falls on the preposition, and the prepositional phrase is adverbialized, i. e., functions as an adverb, hence the stress pattern, since adverbs normally bear the phrasal stress when immediately preceded by the verb they modify (e. g., go quickly, write slowly, breathe deeply, etc.). This also happens when the preposition is a compound, as in look up to him, the stress falling invariably on the first component of the compound.

With first or second person pronouns, stress on the preposition is facultative, whereas with the third person pronoun it, it is obligatory. This pattern is to be explained by the fact that as the neuter member of the category the third person is less central in the hierarchy of pronominal personhood compared to the first and second persons, hence less capable of bearing the stress in the prosodic structure of adverbialized prepositional phrases.

MICHAEL SHAPIRO