For the whole law is fulfilled in one word:
You shall love your neighbor as yourself.
The whole passage (verse 13-20) talks about how the flesh works against the Spirit and vice versa.
To live by the flesh means to run after your own desires, to only think about yourself (egoistic) and to only look for your own best in every situation(you love yourself). Life by the flesh is to love yourself in that way.
But since living by the Spirit is the opposite, it means that in every situation you look for the best of those people around you and don't consider your own profit. Living by the spirit is loving others the same way you loved yourself, when you were still walking in the flesh.
That's exactly what verse 14 talks about. Just like you love yourself when you are egoistic, the same way should you love your neighbors.
You have to stop looking for your own success and benefits and instead look for opportunities to do something good to your neighbors.
Only if you live like that, you have fulfilled the whole law and are therefore free form it. (verse 14 and 18)
If you still find strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, sexual immorality,…(verse 19) in your life though, then it's because you still live by the flesh and haven't perfected in love yet.
All those are manifestations of the flesh, but if you live in complete love and therefore by the Spirit, you will experience exactly the opposite in your life!
Only when you live by the Spirit, in true love to your neighbors, will you be able to enter Gods Kingdom (His system) and live in all His blessings here on the earth!
Walk by the Spirit and not by the flesh!
Walking by the flesh is loving yourself, so that everything you do is for your own best, not even considering others.
Walking by the Spirit on the other hand is loving others that exact same way that you loved yourself, when you were still walking by the flesh.
Don’t allow your flesh to rule over you. Be Spirit-lead and use the love you have for yourself to love your neighbors! (Galatians 5,13-25)