It just seemed that anytime I get in this situation, I'm always behind but it just seems so weak/tight to fold. When does the person have TPTK or the flush draw and when does he have a set? This opponent was slightly loose but aggressive (I believe PFA of 4.0).

Party Poker No-Limit Hold'em, $0.5 BB (9 handed)

SB ($52.25)
Hero ($77.03)
UTG ($50)
UTG+1 ($54.55)
MP1 ($49.4)
MP2 ($74.55)
MP3 ($29.35)
CO ($75.78)
Button ($48.75)

Preflop: Hero is BB with , .
3 folds, MP2 calls $0.50, 2 folds, Button calls $0.50, 1 fold, Hero raises to $3, MP2 folds, Button calls $2.50.

Flop: ($6.75) , , (2 players)
Hero bets $5, Button raises to $15, Hero raises to $74.03 (All-In), Button calls $30.75 (All-In).

Turn: ($126.53) (2 players, 2 all-in)

River: ($126.53) (2 players, 2 all-in)

Final Pot: $126.53

Results in white below:
Hero has Kc Kd (one pair, kings).
Button has 3s 3h (three of a kind, threes).
Outcome: Button wins $98.25. Hero wins $28.28.

This is one of those "I have KK, he must only have the queen so I'll go all in" thoughts I had that were wrong.