The Rambam (Hilchus Ishus 15:19) writes:
כן צוו חכמים שיהיה אדם מכבד את אשתו יתר מגופו ואוהבה כגופו
The Chachamim instituted that [each] man should respect his wife more than himself, and love her as [he loves] himself.
When it comes to the laws of loving others, the verse states:
ואהבת לרעך כמוך
You shall love your friend like [you love] yourself.
What practical difference is there between the obligation to love a wife and the obligation to love a friend, when they are both defined as 'like yourself'? (In other words, what did Chazal add by instituting the obligation to 'love your wife' which wasn't already included in 'love your friend'?)