Doctor Strange duty is to save Earth from outer forces. ( His origin movie talks about that)
Thor is killed as per Bruce, giving OG Avengers a motivation.
Thanos wants Vision dead giving Scarlet Witch a strong motivation to fight.
Avengers have already seen a alien invasion obviously they don't want another one.
Iron Man is always afraid of Alien forces as shown in vision in AOU.
Thor brother is killed.
Gamora doesn't want Thanos to win.
Thanos abducted Gamora, making GOTG a Important part of the revolt against Thanos.
What more motivation do you want?How can they have personal agenda against Thanos when they never met him before.