You knew when this started that this verse would be in the mix somewhere - and if you've figured out that we're going through Scripture book by book - then you probably knew we were headed here soon. You're smart that way.
This is the one that turned my thinking around - oh - about twenty years ago. The fact says "For God so loved the world ..." really stopped me in my tracks one day. It doesn't say for God was so pissed at the world, or for God was so sick of the world for messing up - or fed up, or impatient ... I'm pretty sure there's not one typo in this whole verse.
If you'd asked me back then to list off everything you needed to figure out in order to be as Christian a Christian as you could be - I'm pretty sure I'd have handed you a lengthy list of scary stuff. I can say this pretty safely because I still have all my journals from that back then - and they have loads of such lists.
Along our journey of faith many things can be helpful - but there's no motivation for chasing after God - after being transformed into someone who looks like Jesus like love. Every other motivation will go weird on you. Love however - is self-calibrating along the way - the more we love God - the better we get at loving Him, each other - even ourselves. If you read the New Testament - you realize that there's an important link between obedience and love; but God's motivation was love. If we obey because we love - we'll grow our faith healthy and strong and we'll realize that obedience can teach us much about love. It doesn't work for us to motivate obedience to Jesus out of fear any more than it works to raise kids in fear.
Maybe this is why this was Jesus' motivation for going through it all. I used to debate with a friend about just how loving God is or isn't ... and I'd frequently ask ... "well ... if it's not Love that motivated Jesus - what else could make someone willing to go through such a thing as incarnation, a difficult life, and then a torturous terrible death?"
There wasn't another answer for Jesus.
There's no better answer for us either.
If this motivation hadn't been everything for Jesus - we'd have no Christmas - and we sure wouldn't have Easter. We wouldn't have anything.