Learn Turkish: Hayalperestler neden her zaman mutlu? Çünkü iyimserler. - Why are dreamers always happy? Because they are optimistic.

When a plural human subject is explicitly stated in the sentence (like hayalperestler), adding the plural suffix to the predicate (mutlu) is optional and often left out for better flow. However, in the second sentence (Çünkü iyimserler), the subject 'they' is dropped. Without an explicit subject, you need the -ler suffix to show you mean 'they are optimistic' rather than 'he/she is optimistic'.

What does the -perest part of hayalperestler mean?

It is a Persian loan suffix meaning 'worshipper' or 'devoted to'. Combined with hayal (dream/imagination), a hayalperest is literally someone devoted to their imagination—a dreamer!

How is the word iyimser (optimistic) formed?

It is formed by adding the suffix -msAr to the root word iyi (good). This suffix indicates a tendency or inclination towards something. You will see the exact same pattern in its opposite: kötümser (pessimistic).

How would we add the contrastive topic marker -(y)sA to this sentence to mean 'As for dreamers, why are they always happy?'

You would attach it directly to the topic you want to contrast. If you had just been talking about realists, you could shift the focus to dreamers by saying Hayalperestlerse neden her zaman mutlu? (or writing it as a separate word: Hayalperestler ise...).