Spring is nice, but I feel like it IS a bit of a mixed bag. For those of us that love skiing, it's warm and sunny and thrilling in its own way when we can ski just a wee bit longer into the "shoulder season." At the same time however, it is spring after all, and the skiing can just as easily suck eggs. On the other side of the equation - for those winter haters that are desperate for sun and warm - spring is enticingly fickle. It feels warm and alluring... but it can just as equally suck eggs. And thusly, the "discontent."
From the idea development department, "Now is the winter of our discontent" is the opening line from Shakespeare's Richard III (1592), and it actually translates with a little more context into something like: the time of unhappiness in past. So I guess, but saying that spring is a season of discontent and dashed expectations, I'm probably using it incorrectly. Alas, so it goes. Interpret as you see fit!