Would you translate this: Every day was fun.
Every day became fun.
I know that なる can be used to say that A becomes B. But in this example what is the nuance of the meaning of なる？
毎日は楽しくなりました generally makes it seem like the context is:
1) everyday life was not much fun before
2) a particular thing happened and pretty much changed it over night
3) everyday life is now fun.
Thus, it's difficult to actually translate this into an English sentence that contains all that context. Something like "everyday life became fun".
As for Kdansky's post above, the てしまった form could add more emphasis on the fact that "yeah, it really DID become fun". However, the てきた form that he used implies something slightly different: that it wasn't a "particular thing" that changed the situation "overnight", but rather that everyday life steadily became more enjoyable, and now it is fun.