We present an analysis of a treebank of spontaneous English dyadic conversations, investigating whether the degree of syntactic priming found across speakers is a function of the degrees of affective alignment and overall positivity of the speakers. We use information theory to measure the proportion of overlap between the syntactic structures of the speakers. The affective state of the speakers is indexed by aggregated measures of the affective valences of the words they use. We find that there is a positive relation between syntactic priming and affective alignment, over and above any lexical repetition effects. This constitutes evidence for the percolation of inter-speaker alignment across multiple levels of representation. This also illustrates the indexical value of syntactic alignment, as has been proposed in modern functional theories of grammar such as Dialogic Syntax.