Dr. Shauna H. Springer calls the first phase the cocaine-rush-phase. This is the phase in which we know very little about the other person. According to the fundamental attribution error theory, we should be making negative personality attributions for the other's perceived negative behavior. If acting like a jackass, then one is likely to blame it on the situations he is in currently. When treating with kindness, one is likely to attribute that to his character.
The next phase is what Dr. Springer calls the testing phase. We begin to see our partners in a harsher light in this phase. Negative traits and flaws start to appear. Views of each other begin to change, maybe creating instability. After enough time has passed, perceptions of the other should stabilize and the partners should be able to make an informed decision about whether they truly are a good fit for each other.