Europe

Pole here: Poland joined for four reasons:

1. It is pretty much mandatory if you want to join EU (note that we had referendum about joining EU but nobody asked us if we want to join NATO)

2. You can always count on Poland trying to become western Europe, it's a constant for over 1000 years by now, we join all institutions located there is possible. It's also combined with massive inferiority complex.

3. Russophobia is alongside anticommunism the absolute ideologic pillar of current Polish state, it's basically reflexive at this point. Fun fact: if anything, it's a proof of how even in the 90's NATO was clearly still aimed against Russia.

4. Last but not least, opportunism, NATO totally looked as the victor back then, hell it still looks like this when you're in the bubble.