Aşk bazen insanı üzer.

A word-by-word breakdown is:

• aşk = love
• bazen = sometimes
• insanı = the person / a person / people in a general human sense, as the object
• üzer = upsets, saddens, causes distress

So the structure is basically:

• Aşk = subject
• bazen = adverb
• insanı = object
• üzer = verb

Because insan is the direct object of the verb üzmek.

In Turkish, direct objects are often marked with the accusative ending when they are specific or grammatically treated as a definite object. Here, insan + ı = insanı.

So:

• insan = person / human
• insanı = the person / the human, as a direct object

In this sentence, insanı is used in a generalizing way, something like a person or people/human beings in general.

This is a common point of confusion for English speakers.

Turkish sometimes uses singular nouns to express a general truth about people. So insanı here does not necessarily mean one specific person in a real-world sense. It can mean a person, the human being, or people in general.

Compare the English idea:

• Love sometimes upsets a person
• which can really mean
• Love sometimes upsets people

So the singular form in Turkish can still have a broad, generic meaning.

Üzer is in the aorist form, which is often used for:

• general truths
• habits
• repeated actions
• things that happen sometimes or usually

Here it works like English upsets or can upset in a general statement:

• Aşk bazen insanı üzer = Love sometimes upsets people / Love can sometimes make a person sad

This is why it is not in a clearly ongoing form like üzüyor.

The dictionary form is üzmek, meaning to upset, to sadden, to distress.

Its aorist stem is üzer for third person singular.

Very roughly:

• üzmek = to upset
• üz-er = he/she/it upsets, or in general upsets

So in this sentence:

• Aşk ... üzer = Love ... upsets

You do not need a separate word for it because the subject aşk already tells you what is doing the action.

Because üzüyor means something more like is upsetting or is making sad right now, in an ongoing sense.

Compare:

• Aşk bazen insanı üzer = Love sometimes upsets people.
  A general truth.

• Aşk insanı üzüyor = Love is upsetting people / Love is making one sad.
  More immediate or situational.

Since the sentence expresses a general statement, üzer is the natural choice.

The basic order here is:

• Aşk = subject
• bazen = adverb
• insanı = object
• üzer = verb

So:

Aşk bazen insanı üzer

This is a very natural Turkish order, but Turkish word order is somewhat flexible. For example, you could also hear:

• Aşk insanı bazen üzer

That still means basically the same thing, but the emphasis shifts slightly. Turkish often puts the verb at the end, while other elements can move around depending on focus.

Not exactly a separate hidden pronoun, but Turkish verbs show person, and the subject can often be understood without an explicit pronoun.

In this sentence, though, the subject is already stated:

• Aşk = love

So üzer simply agrees with a third-person singular subject:

• aşk ... üzer = love ... upsets

You do not need to say o.

Usually aşk strongly suggests romantic love, passion, or being in love, more than the broad English word love sometimes does.

So this sentence often feels like:

• romantic love sometimes hurts people
• being in love can sometimes make a person sad

If you wanted a broader idea of love in general, Turkish might use other wording depending on context.

No. This is an easy thing to wonder about because they look similar.

Here, üzer comes from the verb üzmek = to upset.

But in words like:

• üzerinde
• üstünde

the meaning is about on, over, or on top of.

So these are different words with different origins and meanings.

Yes, you could, and it would mean Love sometimes upsets people more explicitly in the plural.

The difference is:

• insanı = a person / the human being / people in a general human sense
• insanları = people, in a more clearly plural sense

Both can work, but insanı has a slightly more general, proverbial feel here.

It sounds quite natural and also a little reflective or aphoristic, like a general observation about life.

It is not overly poetic, but it does have the feel of a short truth or saying:

• Love sometimes upsets people

So it works well in normal Turkish, especially in writing, conversation, or emotional commentary.