Why am I such a weak-tight nit??? Is checking behind on this river just sinful?

Full Tilt Poker Game #893898003: FTOPS Event #1 (5628129), Table 158 - 80/160 - No Limit Hold'em - 19:32:30 ET - 2006/08/13
Seat 1: Bluffem23 (10,380)
Seat 2: KramerTM (9,180)
Seat 3: TomC_Vegas (6,180)
Seat 4: DexterCain (2,050)
Seat 5: composer24 (7,320)
Seat 6: npa (2,485)
Seat 7: steelerjay (3,011)
Seat 8: AZDrRX (4,180)
Seat 9: SoccoNJ (9,145)
npa posts the small blind of 80
steelerjay posts the big blind of 160
The button is in seat #5
*** HOLE CARDS ***
Dealt to KramerTM [Jc Jd]
AZDrRX folds
SoccoNJ folds
Bluffem23 raises to 320
KramerTM raises to 1,100
TomC_Vegas folds
DexterCain folds
composer24 folds
npa folds
steelerjay folds
Bluffem23 calls 780
*** FLOP *** [9s 7s 3h]
Bluffem23 checks
KramerTM checks
*** TURN *** [9s 7s 3h] [7c]
Bluffem23 checks
KramerTM bets 1,455
Bluffem23 calls 1,455
*** RIVER *** [9s 7s 3h 7c] [4s]
Bluffem23 checks
KramerTM checks
*** SHOW DOWN ***
KramerTM shows [Jc Jd] (two pair, Jacks and Sevens)
Bluffem23 mucks
KramerTM wins the pot (5,350) with two pair, Jacks and Sevens
*** SUMMARY ***
Total pot 5,350 | Rake 0
Board: [9s 7s 3h 7c 4s]
Seat 1: Bluffem23 mucked [6s 6h] - two pair, Sevens and Sixes
Seat 2: KramerTM showed [Jc Jd] and won (5,350) with two pair, Jacks and Sevens
Seat 3: TomC_Vegas didn't bet (folded)
Seat 4: DexterCain didn't bet (folded)
Seat 5: composer24 (button) didn't bet (folded)
Seat 6: npa (small blind) folded before the Flop
Seat 7: steelerjay (big blind) folded before the Flop
Seat 8: AZDrRX didn't bet (folded)
Seat 9: SoccoNJ didn't bet (folded)