I just wanted to add another more technical answer for those who might want a more thorough explanation than just, "this is the way we do it."
Definiteness markers like the actually do quite a few things to the meaning of a noun phrase:
(1) I fed cats yesterday.
(2) I fed the cats yesterday.
The main difference between (1) and (2) is that while I can use (1) to describe any random cats, (2) requires me to be talking about some cats that are identifiable -- that is, both I and the people I'm talking to can guess which specific cats they are. (If context doesn't specify otherwise, this usually refers to the cats that I own.) Let's call this property identifiability.
While identifiability is an important difference between phrases that use the and phrases that don't, when you say life is beautiful, life is in fact something identifiable. We all experience it and know of the concept, and clearly, using it without the article makes it very obvious what you're talking about. That is, it shouldn't be ambiguous what exact idea it refers to, unlike cats in (1). So why is it ungrammatical to use the?
The reason this might be difficult for a lot of English learners is that many languages have their version of the stop at identifiability. The English the, however, has extra requirements:
(1) I like cheese.
(2) I like the cheese.
(3) I like the cheese that tastes like mushrooms.
I could just randomly say sentence (1), referring to anything that can be called cheese (which is something most people can certainly identify), but sentence (2) doesn't work unless I'm literally pointing to some cheese, or the context implies there's only one cheese I could be talking about. Yet, sentence (3) is once again grammatical when spoken without context.
Native speakers will say that (2) only makes sense when the person who said the sentence and the people they're saying it to both know of a particular cheese in the context that stands out from all other cheese. For example, this might come up if they're critiquing a cheeseburger, but not if the person talking just wants to say they like cheese in general (e.g. more than peanut butter.) This is a stronger requirement than identifiability -- you don't just need to be able to pick something out, the thing needs to stand out by itself. (2) doesn't have anything making it stand out, so it relies on the context (such as the cheeseburger) to make it grammatical. However, (3) has the cheese naturally narrowed down by the language around it, so even though it refers to a general category like (1), it still stands out from other cheese.
The life is beautiful is just like case (1). The default context is everything, so unless, like jonathanjo brings up, there is a particular life that the context makes special, life simply does not stand out enough for the.