well stajerc it doesn't have to be like this. You might have a dialect which is formed at the border of two tribes. The part of the population speaking the dialect gets separated in some way from the two main tribal groups. You can have invasions of a third tribe which settles between the dialect are and the main tribe group. Or you might have a migration of the clan which speaks the dialect away from the main tribal group into a territory of another tribe. Or you could have a weather or seismic event which cuts the dialect group away from the rest of the tribe…You can also have a religious or political divisions placing borders between the dialect group and the main group…

Yes, but if it's in their interest they can make this dialect into a language, even if it's almost the same or very very similar. I mean, Tyrolean and Platdeutsch are both considered dialects of the same language, but if you'd put a guy from Tyrol and Mecklenburg together they'd have a hard time understanding each other if they'd speak in their dialects. On the other hand Dutch and Flemish are considered languages.