Although you tell yourself you wont start saying 'Propa nice', it cant be helped and is addictive.
The term is used to portray a feeling of pleasure, contentment and approval amongst other positive things.
It is used often in the case of drugs.
Can be reversed to mean something negative by saying 'propa not nice'.
B - "Aw, propa nice man!!"
A - "I had a bad trip from those shrooms last night dude!"
B - "Aw, propa not nice man..."