Mood is contagious at close range and over very short timescales. Speak to someone at a till in a decent tone and you will usually get a decent tone back; arrive tense and the exchange goes flat before either of you has decided anything about it.
Multiply that by the forty small contacts of a normal day and it stops being mysterious. It is closer to weather in a room, and you are one of the things generating it.
The day itself does not rearrange. The people in it respond, and their responses are most of what a day turns out to be made of.