Trust seems to be a "currency" in my opinion. You basically get opportunities to spend trust to raise lust or love further, so I try to keep Trust around ten and spend it to raise other stats when it is higher.
That's probably true to a certain extent. However, there are actions that will raise Trust along with one of the other (or, rarely, both of the other) stats. Trust seems to be more of a stat to determine whether you're able to get a girl like Jodie to do certain things she otherwise might not. It might affect, for instance, whether pimping a girl out will have less of a (or any) negative impact on you. In cases of acts of extreme Lust or Love,
those stats themselves are often the prerequisites.
I think Trust really is just that: trust. It appears to be tied to a girl accepting something you do or say, interacting with other people, doing things she otherwise would never be willing to do, etc.