It has to do with semantic nuancing. Take a look here, page i.
Mitigation means to weaken. In the examples below, adding <y> to a vowel weakens the word's meaning-- mitigates the sense.
Examples of vowel mitigation____________________________
ayma "warm" <de-intensified>
ey "half" <de-intensified>
mayr "star" <de-intensified>
ifer "to love"
iyfer "to like" <de-intensified>
See also this source http://goliath.ecnext.com/coms2/gi_0199-6705243/Aggravation-and-disagreement-a-case.html. Here's a part:
Disagreement or opposition in interaction where agreement is preferred may be mitigated or aggravated. Mitigation is the softening of the threat to the other's face. The action carried out in the disagreement is minimized or disguised through mitigation, which Caffi (1999) divides into three strategies: bushes, hedges and shields. Bushes make the propositional content of the speech act less precise, hedges weaken the illocutionary force of the speech act, and shields displace the disagreement's origin away from the speaker (Caffi 1999). Interactants employ a variety of linguistic resources to mitigate, including turn-taking choices (e.g. delays, silence), morphological choices (e.g. diminutives, tense/mood/aspect), and lexical choices (e.g. register, specialized vs. general vocabulary). In situations where disagreement is dispreferred, mitigation may be used to mask a disagreement, making it, if not unnoticeable, less noticeable and, therefore, less accountable and sanctionable.