Economy: Speakers tend to make their utterances as efficient and effective as possible to reach communicative goals. Purposeful speaking therefore involves a trade-off of costs and benefits.
the principle of least effort: Speakers especially use economy in their articulation, which tends to result in phonetic reduction of speech forms. See vowel reduction, cluster reduction, lenition, and elision. After some time a change may become widely accepted (it becomes a regular sound change) and may end up treated as a standard. For instance: going to [ˈɡoʊ.ɪŋ.tʊ] → gonna [ˈɡɔnə] or [ˈɡʌnə], with examples of both vowel reduction [ʊ] → [ə] and elision [nt] → [n], [oʊ.ɪ] → [ʌ].
Analogy: reducing word forms by likening different forms of the word to the root.
Language contact: borrowing of words and constructions from foreign languages.
The medium of communication.
Cultural environment: Groups of speakers will reflect new places, situations, and objects in their language, whether they encounter different people there or not.
Migration/Movement: Speakers will change and create languages, such as pidgins and creoles