The monograph Motivation problems of poly-lexical stable units in Romanian by the researcher Angela Savin-Zgardan demonstrates that the language system is characterized by complexity and contradiction, qualities that make the essence of a poly-lexical stable unit (PSU) as a dialectical unit of systematicity and non-systematicity of a language. Unlike the statements made by of F. de Saussure, Ch Bally, S. Ullman S. Ullman in which they distinguished a non-motivated character of vocabulary, the author of the study states that the internal form of set expressions which appear as plastically-motivated units of language, is clearly felt by the speakers, which gives them a possibility to express their attitude towards the given units even in their potential state. The speaker’s positive or negative reaction can determine a stable combination of words to become a language unit. Here's why in the formation of stable combinations subjective (ethno-linguistic and ethno-psychological) factors play a much larger role than in word-formation. The study argues, by highlighting and describing extralinguistic and intralinguistic factors, the motivation of emergence, existence and use of PSU in Romanian.