Here's another reason why you want to raise Pocket Pairs. Villain not raising his 55 cost him his stack :

Party Poker No-Limit Hold'em, $ 50 (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Hero ($57.75)
UTG ($82.40)
MP ($60.38)
CO ($68.68)
Button ($17.16)
SB ($58.25)

Preflop: Hero is BB with 6, 3. SB posts a blind of $0.25.
UTG calls $0.50, MP calls $0.50, CO calls $0.50, 1 fold, SB (poster) completes, Hero checks.

Flop: ($2.50) 5, 2, 4 (5 players)
SB bets $1.5, Hero calls $1.50, UTG folds, MP raises to $6.61, CO folds, SB folds, Hero raises to $12, MP calls $5.39.

Turn: ($28) A (2 players)
Hero bets $20, MP calls $20.

River: ($68) 7 (2 players)
Hero bets $25, MP calls $25.

Final Pot: $118

Results :
Hero has 6d 3d (straight, seven high).
MP has 5s 5h (three of a kind, fives).
Outcome: Hero wins $118.