Not sure of SouthWest Airline rules today, but some time ago I had a flight booked with them and they didn't provide seat assignments. First come, first served but didn't know this! I was early (first at the gate) assuming I would get a seat, waiting for the desk at the gate to be staffed to obtain a seat assignment. No one came but noticed people lining up early so became concerned. Eventually the desk was staffed so I cut in to the front to get my seat assignment. You can imagine the outcry. Told everyone within ear shot I had been there since the previous flight disembarked. Fortunately, they believed me and I was able to go to the front and board the aircraft. Left a very bad taste. Will never book a Southwest Airlines flight again, although no other issues and the crew announcements were entertaining.
Maybe different on SW Airlines today, but as accepted rules and common courtesies seem to have gone out the window, for whatever activity I engage in, I will purchase a ticket and solidify the seat assignments BEFORE leaving the house. And yes, sadly I am aware of situations where even assigned seats are fought over causing much distress to both cabin crew and other travelers.