Topic notes, not a full sermon. Take the structure from the sermon outline page and the material from here.
Three texts, three costs
choose one[each one makes a different sermon - do not take all three]
1 John 4:7-21. The strongest text and the hardest to keep concrete. Its claim is that love is not something God does but something God is, which is enormous and slides into abstraction within three minutes unless you anchor every paragraph in something physical.
1 Corinthians 13. Everybody knows it and almost nobody knows it was written to a church tearing itself apart over spiritual status. Preached with its context restored it stops being a wedding reading and becomes a rebuke, which is far more interesting.
John 15:12-17. Narrow and concrete: lay down your life for your friends, said by someone about to do it that week. Costs you breadth and buys you weight.
The one-word problem
the useful distinction[use it, do not structure the sermon on it]
English asks one word to carry what a person feels about their spouse, their children, their friends and their football team. Greek spread that load across several. Which means the congregation hears a word that is mostly about feeling, and the text usually means something mostly about commitment.
That gap is the sermon. Say it once, in a sentence, and then spend the rest of the time on what commitment looks like on a Tuesday.
The tension worth naming
the honest bit[every congregation has someone for whom this is not theoretical]
Being told to love is useless to someone who is trying and failing to feel it about a particular person. That is the situation most people are actually in when they hear this sermon, and it is the one most love sermons never address.
If love is something you do rather than something you generate, then a person who acts well towards someone they cannot stand is not a hypocrite. They are doing the thing. Say that plainly and watch the room.
An angle that is not the obvious one
if you want it[for a congregation that has heard the standard version many times]
1 John says we love because he first loved us. The order is the claim. Most of our trying runs the other way round, attempting to generate enough love to deserve being loved, and 1 John says that sequence is impossible and unnecessary.