He was a constantly energetic person with a wacky personality who was cheerful, talkative and looked on the bright side of everything, but unfortunately for Phoebe also had a strange and annoying habit of constantly complimenting the most trivial things in a ridiculously over-the-top way.
At one point after complimenting brake lights he described himself as "putting a good spin on a traffic jam," indicating that he actually knows how trivial the things he is complimenting are, but feels the need to "put a good spin" on them nonetheless. He was invited to Jack and Judy Geller's 35th wedding anniversary, but he spent the entire night excitedly making a big fuss of everything regardless of how trivial whatever it was he was complimenting was, which Phoebe eventually began to find exhaustively annoying.
After witnessing and being bemused by Monica's ridiculous failed attempt at a speech, Parker went to Phoebe's apartment with her and complimented her hallway and then her sofa in his usual over-the-top way, and when Phoebe, who is tired of his constant enthusiasm by now, tries to get him to calm down by suggesting they play a no talking game, he responds by cheerfully saying "I lose, now (let's play) Jenga." Unable to take it anymore, Phoebe snaps and shouts at Parker, and when she tells him she wants him to be "much less happy" he breaks up with her...and after angrily storming out of the room, he even returns to compliment the argument.
Although the friends remind themselves throughout the episode that in spite of his annoying nature, Parker seems to be a nice guy, his attitude towards his break-up with Phoebe shows that he never really cared about or took their relationship seriously, even though he claimed when he met her for the first time in the drycleaner's that he believed they were meant to be together and allowed her to believe this herself until they broke up-presumably, so that she would sleep with him. This implies that Parker actually wasn't such a nice guy after all-outwardly friendly, but inwardly very selfish and also rather manipulative.
Given Parker's constant enthusiasm, inability to calm down or keep his attention on any one thing for long such as his anger towards Phoebe after their argument and break-up, impulsive behavior such as putting his hand on the pregnant Rachel's stomach before she could give him permission when he asked for it, badgering Phoebe into eating an oyster despite her protests before refusing to eat one himself because they "looked too weird" and giving Chandler a friendly punch on the arm with so much energetic force that he accidentally hurt him in the process, and obliviousness, failing to notice he was irritating people and that he'd hurt Chandler, he most likely unknowingly suffered from attention deficit hyperactivity disorder. However, his perfectly calm demeanor when he and Phoebe met for the first time at the drycleaner's and the fact that she had enjoyed his company and never found him to be too enthusiastic prior to the night he accompanied her to the party suggests that he isn't usually so over-excited about everything, making it safe to speculate that prior to meeting up with her and her friends he may, perhaps in an attempt to combat nervous feelings about going to a place he'd never been to before and being surrounded by so many people he'd never met before, have been drinking that night.