Are you losing money, or winning information?

Sure you lost the hand when your opponent managed to hit his card on the river, but now you have the knowledge that he/she will pay you off the next time and the next time and next time.

A slightly twisted view of this is watching Daniel Negreanu on the WPT or WSOP calling a river bet despite suspecting that he's beat and calling it "paying for information". Then using the information later on and profiting from it.

PokerStars Game #8053915391: Omaha Hi/Lo Pot Limit ($0.50/$1.00) - 2007/01/21 - 14:53:36 (ET)
Table 'Kira' 6-max Seat #6 is the button
Seat 1: sparksny1 ($48.35 in chips)
Seat 2: Drizztdj ($124.35 in chips)
Seat 3: BRT123 ($61.25 in chips)
Seat 4: Dutchy888 ($16.40 in chips)
Seat 5: bakota_tokyo ($36 in chips)
Seat 6: df1986 ($96.15 in chips)
sparksny1: posts small blind $0.50
Drizztdj: posts big blind $1
*** HOLE CARDS ***
Dealt to Drizztdj [Jh Js Ks 9h]
BRT123: calls $1
Dutchy888: folds
bakota_tokyo: folds
df1986: folds
sparksny1: calls $0.50
Drizztdj: checks
*** FLOP *** [Qc 2s 8d]
sparksny1: checks
Drizztdj: checks
BRT123: checks
*** TURN *** [Qc 2s 8d] [Ts]
sparksny1: checks
Drizztdj: bets $2
BRT123: raises $4 to $6
sparksny1: folds
Drizztdj: raises $14.85 to $20.85
BRT123: raises $39.40 to $60.25 and is all-in
Drizztdj: calls $39.40
*** RIVER *** [Qc 2s 8d Ts] [2d]
*** SHOW DOWN ***
Drizztdj: shows [Jh Js Ks 9h] (HI: a straight, Eight to Queen)
BRT123: shows [Ah 8h Td Th] (HI: a full house, Tens full of Deuces)
BRT123 collected $120.50 from pot
No low hand qualified
*** SUMMARY ***
Total pot $123.50 | Rake $3
Board [Qc 2s 8d Ts 2d]
Seat 1: sparksny1 (small blind) folded on the Turn
Seat 2: Drizztdj (big blind) showed [Jh Js Ks 9h] and lost with HI: a straight, Eight to Queen
Seat 3: BRT123 showed [Ah 8h Td Th] and won ($120.50) with HI: a full house, Tens full of Deuces
Seat 4: Dutchy888 folded before Flop (didn't bet)
Seat 5: bakota_tokyo folded before Flop (didn't bet)
Seat 6: df1986 (button) folded before Flop (didn't bet)

I found out this guy would call off his stack with middle set despite a draw heavy board. Later on, I used this information to value bet him with non-lock hand and snatch back his ill-gotten gains. Easy way out would have been to start making remarks about his mother doing stripteases in the third bathroom stall at McDonald's for a six piece McNugget happy meal.

Instead I waited him out and walked away with a little profit later on.