For years (I think since the original iPhone), Messages only occasionally displays timestamps to mark general milestones in conversations, but not for each individual message. Presumably, this is to cut down on visual clutter.
Messages in iOS 7 maintains this practice and layout, but now you can swipe to the left in a message to show a column of timestamps for every single message. However, keep your finger on the display after swiping if you want to review the stamps; once you let go, the column disappears.