MHIG, but why?

These hands bother me.

***** Hand History for Game 2061100970 *****
$50 NL Hold'em - Monday, May 16, 23:49:03 EDT 2005
Table Table 36597 (Real Money)
Seat 5 is the button
Total number of players : 10
Seat 1: pkrzyn ( $46.95 )
Seat 7: Warrior2323 ( $134.2 )
Seat 9: The_Greek_1 ( $199.55 )
Seat 6: sejje ( $48.5 )
Seat 10: dales0045 ( $50 )
Seat 2: PreemPalver ( $38.25 )
Seat 3: njboy456 ( $45.44 )
Seat 8: yunsoonmp ( $49.6 )
Seat 5: WiCane ( $39.95 )
Seat 4: mth03d ( $76.6 )
sejje posts small blind [$0.25].
Warrior2323 posts big blind [$0.5].
** Dealing down cards **
Dealt to sejje [ As Ac ]
yunsoonmp folds.
The_Greek_1 calls [$0.5].
dales0045 folds.
pkrzyn folds.
PreemPalver folds.
njboy456 folds.
mth03d folds.
>You have options at Table 37425 Table!.
WiCane calls [$0.5].
sejje raises [$1.75].
Warrior2323 calls [$1.5].
The_Greek_1 calls [$1.5].
WiCane folds.
** Dealing Flop ** [ 6h, 4h, 9d ]
>You have options at Table 36586 Table!.
sejje bets [$6].
Warrior2323 folds.
The_Greek_1 calls [$6].
** Dealing Turn ** [ 5s ]
sejje bets [$12].
The_Greek_1 calls [$12].
** Dealing River ** [ 3d ]
sejje checks.
The_Greek_1 checks.
sejje shows [ As, Ac ] a pair of aces.
The_Greek_1 doesn't show [ 5d, Ah ] a pair of fives.
sejje wins $40.4 from the main pot with a pair of aces.

These kinds of hands I never understand what I should have done. Should I have bet out? I would have folded to much of a bet, and wrongly apparently.

They guy was playing pretty well. I just odn't understand.

I would probably have checked the river and called a small-medium bet like you were planning. Unimproved AA on the river isnt anything to get another stack allin.

It looked like he was playing for that pair after the turn because I dont see any possible straight draw for him to be drawing too. He may have called another river bet of 12 or something, but its hard telling.

I think you did with AA just what is supposed to be done. You played it aggressively and won a pretty decent pot without much difficulty.

I don't know what he was thinking there. He had no reason to call that bet on the flop; what was he hoping for, a runner-runner flush or straight? The turn only marginally improved his hand. I can understand you being baffled by this; at no point did he have a compelling reason to call your bets.

Had you been PFR/continuation betting a lot in the session with lots of holdings? Maybe he was waiting for you to blink, so he could try to steal, or just locked you on a bluff.

This situation always makes me a bit nervous, even with AA, cause you're waiting for him to push all-in with trips on the river.