The "Shift" is when a guy (sometimes a gal) goes from DFS to DFL. Sometimes it happens because he has been dating seriously and sees the long-term potential or possibly has experienced lovematism. Sometimes it happens because he is bored with superficial one-night-stands or FWB and wants to settle down with one woman.
The "Shift" is when a guy leaves the dateship boards the relationship.
Guy One to Guy Two: "Are you going through THE SHIFT? Stop that crazy talk about her being 'the one.'"
Susan to Jane: "Wow, he's so mature for his age to have gone through THE SHIFT."
what irish people call snogging or making out
a fridget has never had the shift
i got the shift last night
he got shifted at school
she has never had the shift