Is Share Spells a Two-Way street?

Share Spells: The wizard may cast a spell with a target of “You” on his familiar (as a touch spell) instead of on himself. A wizard may cast spells on his familiar even if the spells do not normally affect creatures of the familiar's type (magical beast).

If a familiar had the ability to cast a spells, could they use Share Spells to cast a spell with a target of 'You' on their Master?

No. But it's not a bad idea.

It seems like the rule was written during 3.0, back before familiars ever had the capacity to use items or cast spells. I'm confused why it wouldn't work both ways.

It doesn't work both ways because it doesn't say it does. This is the rules forum, "sounds right" or "should work" are no substitute for "what it actually says". And what it actually says is that a wizard can cast certain spells on his familiar even if he couldn't normally.

If you want the fluff, presumably it's because the wizard is placing a piece of their soul in the familiar to give them the spark of intelligence/power/whatever. The familiar is not doing something similar (what does part of an owl's soul in a human do?) so they can't reciprocate. This is why you used to get negative levels for a dead familiar in 3.5, I'd guess.

Fair enough. Guess I'll just add it to my list of house rules.