I missed Oz when he left. I wanted Willow to choose him over Tara. When he left, the fight was all around a decision he had to make to either lock Verruca up or let her out when he knew she would murder. I don't think he had much of a choice and I think he did the right thing but can see how Willow was completely gutted.
Then he left and he found and cure, he endured loads of things for her for months and finally came back to her when he could control the best inside and knew he wouldnt hurt her in his wolf form which he couldnt control anymore. That was a hell of a lot to do for love.
And then he comes back and you just begin to remember how sweet they were together, how he helped Willow become more self assured and less insecure and how they both paniced and slept together at Graduation and how he jumped and got shot for her and just all the really sweet moments they had in season 2 and 3 and 4.
When he comes back I just wish Willow would have just taken him back and they could have lived happily ever after. But she couldn't and now I see that there wasnt really anywhere else for the relationship to go so it would have gotten stale.