Why do I call when I'm so sure I'm beat? (low content)

HH is unedited. I don't understand myself sometimes. It was pretty clear to me what he had...

PokerStars Game #3910180455: Hold'em No Limit ($0.50/$1.00) - 2006/02/09 - 00:01:11 (ET)
Table 'Elara III' Seat #7 is the button
Seat 1: Bealesy ($85.95 in chips)
Seat 2: speedycarlos ($100 in chips)
Seat 3: chrissy25 ($42.40 in chips)
Seat 4: BurgerBoy55 ($50.75 in chips)
Seat 5: artman ($42.85 in chips)
Seat 6: jad925 ($96.60 in chips)
Seat 7: westie44 ($127.50 in chips)
Seat 8: iamapoc ($158.50 in chips)
Seat 9: Lukieplaya ($100 in chips)
iamapoc: posts small blind $0.50
Lukieplaya: posts big blind $1
*** HOLE CARDS ***
Dealt to Lukieplaya [Kd Jd]
Bealesy: folds
speedycarlos: folds
chrissy25: folds
BurgerBoy55: folds
artman: calls $1
jad925: folds
westie44: folds
iamapoc: folds
Lukieplaya: checks
*** FLOP *** [9d 6s Js]
Lukieplaya: bets $2.40
artman: calls $2.40
*** TURN *** [9d 6s Js] [2c]
Lukieplaya: bets $5
artman: calls $5
*** RIVER *** [9d 6s Js 2c] [Ah]
Lukieplaya said, ","
Lukieplaya: checks
artman: bets $7
Lukieplaya said, "ugh"
Lukieplaya said, "nut flush draw hits ace?"
Lukieplaya: calls $7
*** SHOW DOWN ***
artman: shows [7s As] (a pair of Aces)
Lukieplaya: mucks hand
artman collected $29.80 from pot
*** SUMMARY ***
Total pot $31.30 | Rake $1.50
Board [9d 6s Js 2c Ah]
Seat 1: Bealesy folded before Flop (didn't bet)
Seat 2: speedycarlos folded before Flop (didn't bet)
Seat 3: chrissy25 folded before Flop (didn't bet)
Seat 4: BurgerBoy55 folded before Flop (didn't bet)
Seat 5: artman showed [7s As] and won ($29.80) with a pair of Aces
Seat 6: jad925 folded before Flop (didn't bet)
Seat 7: westie44 (button) folded before Flop (didn't bet)
Seat 8: iamapoc (small blind) folded before Flop
Seat 9: Lukieplaya (big blind) mucked [Kd Jd]

Like you said I don't understand either. Happens to me all the time and I have to improve my discipline.

Because making a call where you're going to lose more often than not is sometimes profitable and/or prevents worse forms of exploitation.

The funny thing about this call, is that the bigger bets are more likely to be from worse hands. Once I picked up on this little tip, I started picking off some nice pots.