Villain's likely hand range, from most likely to least likely:
1) Some type of draw (overs + flush, overs + gutshot, flush draw, etc.)
2) A pocket pair lower than yours or 5's that figures you missed
3) A bluff
4) trip 7's

Getting about 3.5-1 vs that range makes this an easy call. Don't be discouraged about the call if he happened to show you 7x this time. Against a tight player this becomes a much tougher decision, but against this player call all day and make $