Let's try.
Aristotle and St. Thomas began with a position of moderate realism from the principle of noncontradiction - the famous "one and the same thing cannot both be and not be in the same time in the same place and in the same manner". Using this axiom they investigated the reality around them and sought correspondences and structures which they then attempted to describe logically and consistently.
This method was more or less objective and self-correcting - sometimes one made mistakes or saw things only from their own vantage point to the neglect of important details, but the more one reflected and the more one looked, the clearer things would become.
The nominalists broke from this, famously, by saying that since the power of observation could be deceived the Thomistic project was inherently subjective and that it is impossible to accurately describe things in general terms.
Descartes accepted nominalist arguments but was unhappy with the nominalist conclusion, so he tried to find some basic datum that could be accurately observed without being deceived by man's fallible senses and he settled upon self-awareness - the famous "I think therefore I am". Just compare an article from St. Thomas' Summa to the first of Descartes Meditations and one can see a tremendous shift from Thomas' dry, impersonal analysis to Descartes highly personalized, novelistic (by comparison) study.
At this point Descartes is saying that the self-aware human person is the arbiter of reality.
Kant goes further by saying that all our sense data are not part of an objective reality, but are merely the way we perceive a reality which cannot ever truly know. Kant moves from considering reality from the vantage point of the subjective observer to that of simply considering the nature of perception, since we can never have contact with the thing-in-itself
Husserl then followed Kant by saying that the thing-in-itself (objective reality) should be abandoned forever and that the thing as it disclosed itself to the observer (i.e. as a phenomenon rather than object) was the important focus.
From there it was only another step to personalism - from how things present themselves to me to the level of how I present my own phenomenological perception of things to myself.
From hardheaded observation of reality to a reality filtered through my own feelings and emotions.