It can be really hard to get seats together for flights during the holidays. Flights are full. More families are traveling together so more people are trying to sit together (compared to solo business travelers). And more and more airlines are holding back the number of seats they assign for ‘free’.
For most passengers, your ticket doesn’t come with a ‘seat’. Obviously that isn’t literally true, since safety rules require all passengers to be seated. But there’s a limited number of seats on the seat map that airlines will let passengers reserve in advance.