Please consider the following sentences:
- It's part of the life.
- It's a part of the life.
What is the difference in the meaning between these sentences?
Why are we allowed to omit the article from the first sentence?
The first thing it say is that you need life and not the life. That’s because you’re referring (I assume) to life in general, rather than to any specific life.
There’s little difference in meaning between part of life and a part of life. Both make generic reference, that is, they refer to part in general, and not to any particular part. The choice between them depends on whether in this case you view part as a singular countable noun, in which case you would say a part of, or whether you view it as an uncountable noun, in which case you would say part of. I would guess that part of was more common.
Both sentences mean similar things but with a slightly different emphasis. The first tends to suggest an important part of the life:-
Being the butt of obscene jokes? It's part of the life [of a pheasant plucker].
Adding the indefinite article adds the connotation that it is a smaller, not quite so important aspect of the life.
Getting tickly feathers down your trousers? It's a part of the life [of a pheasant plucker].
I agree with Barrie England's answer, and I will just elaborate a bit on it, especially in terms of adding examples and also links to dictionary entries.
As Barrie said, the noun part has two kinds of meanings, one countable, the other uncountable, and it so happens that both kinds fit into the construction part of [something], and moreover both mean pretty much the same thing in that context.
That this is so is particularly clear from the entry for part in Cambridge Dictionary, which adds '[C]' or '[U]' next to its noun entries (countable or uncountable). The first entry for part has the disambiguating synonym some. The only definition in this sense is given as '[U] some but not all of a thing'. As examples of usage, we are given
Part of my steak isn't cooked properly.
Part of this form seems to be missing.
I think part of her problem is that she doesn't listen carefully enough to what other people say.
Here part of my steak can be replaced by some of my steak, part of this form by some of this form, and part of her problem by some of her problem (see e.g. here).
There is a second group of meanings, all under the disambiguating synonym separate piece. They all have the '[C]' annotation. As an example, the first of these meanings is '[C] a separate piece of something, or a piece that combines with other pieces to form the whole of something'. One of the examples of usage is given as
I think there's always a part of you that doubts what you're doing.
So, for example,
Struggle is part of life = 'Struggle constitues some of, although not all of, life'
Struggle is a part of life = 'Struggle is a separate piece of life, perhaps a piece that combines with other pieces of life to form the whole of life'
I slightly disagree with Brian Hooper's answer in that I don't think there is any contrast here in terms of how important that part is.
The life vs just life
As many have already commented, when you say the life, you are refering to a specific lifestyle that is presumably clear from context. For example, you might be talking about what's it like to be a pop star, and you might say that constantly dealing with the paparazzi is part of the life, in other words, an unavoidable feature of life of a pop star.
On the other hand, [a] part of life is more or less a set phrase or maybe even a fixed expression. It is possibly also a cliché, especially in such expressions as death is [a] part of life. When we say that some X is part/a part of life, we are saying that this X is something that is regularly encountered in the course of one's life, that there is nothing we can do about it, and that therefore it is pointless to be excessively distressed about the fact that X is so commonly encountered.