Why do some linguists group Baltic and Slavic languages together as Balto-Slavic?

Linguists who group Baltic and Slavic languages together under the Balto-Slavic hypothesis do so because these branches share a significant number of similarities in their vocabulary, grammar, and sound systems that are not found in other Indo-European languages.21

Proponents of this theory argue that these shared features are evidence of development from a common ancestral language following the breakup of Proto-Indo-European. These commonalities include:2

• Morphological and Syntactic Features: Both groups share specific grammatical structures, such as the genitive plural form and the use of the genitive case instead of the accusative with negated verbs.1
• Lexical Similarities: There are more than 100 words common to both Baltic and Slavic in both form and meaning, such as terms for "hand," "smoke," and "linden tree".1
• Phonological Correspondences: The languages exhibit shared patterns in stress and tonal correspondences.1

Conversely, scholars who reject the hypothesis suggest that these similarities are the result of parallel development and mutual influence caused by long-standing, unbroken contact between the two groups.21