“לֹא הָלַךְ בַּעֲצַת רְשָׁעִים וּבְדֶרֶךְ חַטָּאִים לֹא עָמָד וּבְמוֹשַׁב לֵצִים לֹא יָשָׁב…”

“.. that he did not follow the counsel of the wicked, neither did he stand in the way of sinners nor sit in the company of scorners.”

Even the slightest touch with the evil can easily be contagious:

It starts with a small talk, just a glance, one tells himself: ‘what can an open minded chat harm?, just listening to other ideas won’t affect me, with my strong beliefs’..

To start with, you might not ‘follow the counsel of the wicked’, but this is how things will develop:

The ‘quick look’ will become more like ‘a touch’.
What can happen from trying only this once? one will ask himself. ‘I am not changing myself, I am a good guy that only experience what others are doing’.. In the words of the Psalmist: the counsel is now a stand.

The end of the ‘glance’ that became ‘a touch’, is turning from something external to one’s own self. The temporary contact will quickly turn to integration with the soul and dominates it. Now they sit together.

To sum it up,
The cautious from the evil person, is because of the affect they have on one’s soul. The one who knows how to avoid this inner damage, will be a more stable man, therefore happier and ready to the second part, of ‘choosing the good’.
After you avoided the bad, you can chose to be in the good side.
‘Shun evil and do good’ (Psalm 34:15)