Paul models a style of leadership that is focused on the needs of others. He takes his joy from the successes of others and then encourages them to do the same. His joy is in the hands of the Philippians – they can make his joy complete by having the same mind – or attitude – as Jesus Christ. What follows this verse is the Christ hymn, one of the (if not the) earliest recorded Christian hymn, that shows how humble Christ was and retells the Christ story, a story that Paul here uses to encourage the Philippians to be united.
There are some negative consequences of allowing your joy to be determined by the actions of others, but Paul isn’t advocating codependence here, I don’t think. I think he just takes joy in other people bettering their relationship with God, acceping the reality of what Jesus did for us, and then living that out honestly and faithfully. Taking joy in other people like this is humbling – it makes their needs, their journeys, and their lives important. That’s what verse 4 is about: look to the interests of others. Not because you’re unworthy, but because you, like Christ, are so convinced of your self-worth and of God’s love for you that you are freed to lift others up instead of yourself.