Certain regions of the galaxy are more favourable for life, others less. Life takes some time to develop, and it needs chemical elements heavier than helium, e.g. carbon or silicon.
To get sufficient time to develop, the solar system must not be disturbed too much by neigbouring stars. Therefore inner densely populated parts of the galaxy are less suitable than the periphery we are located. (The central star of the planetary system must not be too young, as well, to allow evolution.)
Heavier chemical chemical elements are sufficiently abundant mainly in parts of the galaxy with relatively young stars (population I). This kind of stars is found mainly in the thin disk close to the galactic plane.
Very old stars tend to expand, and may eventually destroy potential life on their planets, if they are surrounded by rocky planets at all.
More discussion on a potential galactic habitable zone on Wikipedia.
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