Except for a few notable styles, the majority of beer is a beverage best enjoyed fresh. It’s a natural product containing a variety of fermented malted grains and some residual yeast and hop matter. While a some larger breweries may choose to pasteurize their beer for longer shelf-stability, most craft beer as a finished product is a little more “alive,” and has instead either rested long enough in the tank (lagered) to be clear, undergone a fining or filtering process or has been centrifuged to remove larger matter that could contribute to a shorter timeline of decent flavor and appearance for you the consumer.
There is a range of temperatures at which we drink beer–usually quite cold for lagers, somewhat less cold for most ales, and warmer for darker, sweeter, or maltier beers. There are great reasons for these choices–not so much rules but guidelines for best flavor. And as I’ve written about previously, storage of beer should be at a stable and cool to cold temperature to prolong the pleasant flavor and life of the beer. Why? A simple answer with a complicated formula to help explain. Essentially, the trouble with warm temperature is not actually the temperature itself but the problematic presence of oxygen, which loves to react with all sorts of things in beer to produce flavors we usually perceive as unpleasant. When a beer is held at a warm or hot temperature, molecules get excited and move around a lot more than at a cool temperature, much like you and I in the park on a sunny day. More movement of molecules means more potential for reactions to occur, of all sorts. This of course isn’t unique to beer, and that complicated formula above can be boiled down to a general rule that the potential for reactions double for every 10 degrees Celsius increase. Obviously, any packaged beer should attempt to contain the lowest amount of oxygen possible (both by minimizing headspace and by measuring and managing dissolved oxygen levels). However, there will always be some oxygen. And thus, there will always been some oxidation. Keeping beer at lower temperatures means that those oxidative reactions will ideally occur slower–and hopefully you’ll drink the beer before most of them.
The other main consideration when talking about temperature in beer is something we call chill haze. This is a totally cosmetic issue and does not change the taste of beer, but it can severely alter your perception and possibly your enjoyment. In a nutshell, certain proteins (from malt and/or hops) make long bonds with polyphenols in beer at low temperatures (35 degrees Fahrenheit or so). These bonds end up somewhat white-ish and generally interrupt the nice flow of light through your regularly clear beer. Chill haze only forms at cold temperatures and will usually disappear when beer is warmed up. However! Too many times moving a beer from cold to warm and back again, or too long at super-low temperatures can make this innocent temporary chill haze a forever haze. Not very pretty. And although it won’t taste any worse for it, chill haze can sometimes be a sign of a brewery issue or a major change in an ingredient’s chemical makeup that either wasn’t anticipated or wasn’t managed–it turns out that brewing chemistry is complicated. There are all sorts of (natural) products breweries use to prevent or fix this issue, but once it’s in your fridge, the only advice I have is don’t keep it ice-cold (for too long) and if you see chill haze, don’t stress! It’s very literally chill.