LAG bets into my middle pair, I spew chips like a moron

[DontRaisePlz]

PokerStars 1/2 Hold'em (6 handed) converter

Preflop: Hero is BB with 6[img]/images/graemlins/club.gif[/img], 7[img]/images/graemlins/spade.gif[/img].
<font color="#666666">4 folds</font>, SB completes, Hero checks.

Flop: (2 SB) K[img]/images/graemlins/club.gif[/img], 4[img]/images/graemlins/club.gif[/img], 7[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">SB bets</font>, <font color="#CC3333">Hero raises</font>, SB calls.

Turn: (3 BB) 5[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">SB bets</font>, <font color="#CC3333">Hero raises</font>, SB calls.

River: (7 BB) A[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
SB checks, Hero checks.

Final Pot: 7 BB

He constantly bets nothing out of position and I wanted to play this hand on the flop but he will call any raise anyway. After betting the turn, I was convinced he didnt have nothing so I checked the river.