Daniel, it was quite an interesting play of fates and luck!

Lovell flew with Gemini 7, then he was given the back up crew for Gemini 10 (which meant commanding Gemini 13 which wasn't going to exist anyway).
See and Bassett (Gemini 9 prime crew) are killed and Lovell and Aldrin are moved to back up Gemini 9 which puts them in line for Gemini 12 which they fly (Aldrin's Apollo 11 seat is thus owed too to See's pilot error).
Then the crew of Apollo 1 dies and Lovell winds up being back up crew (CMP) for Apollo 8 but Collins (prime CMP for Apollo 8) must undergo an operation and Lovell takes his place in Apollo 8.
Lovell was also slated to command A14 but was moved up to A13 because Shepard wasn't ready yet.

Lovell and Cernan had brilliant astronaut careers because they had great qualities: they were commanded by guys who had ways to influence crew selections (Borman and Stafford) and because they always followed their commander's orders.

For me, the second astronaut selection (the Gemini astronauts) had the most interesting guys! (White, Mc Divitt, Stafford, Young, Armstrong, Conrad, Borman, Lovell, See)

Raoul