There is a spectrum here with two ends.
1) Steer clear of doing it consciously
The first is best illustrated by this quote from Hemingway.
No good book has ever been written that has in it symbols arrived at beforehand and stuck in [...] That kind of symbol sticks out like raisins in raisin bread. Raisin bread is all right, but plain bread is better. [...] I tried to make a real old man, a real boy, a real sea, a real fish and real sharks. But if I made them good and true enough they would mean many things. The hardest thing is to make something really true and sometimes truer than true.
The idea is to write a very plain story. Just tell the reader what happened as plainly as you can, and let the symbolism develop itself.
This doesn't mean you end up with a story with no symbolism. The Old Man and the Sea, which he is talking about here, is an incredibly symbolic tale, and after reading it, you can't help but map it onto all sorts of situations. But that's his point: the more natural, deeper symbolism in your story will be a symbol for many different things. You leave the details up to the reader.
The key here, is that by not adding symbols to the story to emphasize what is happening, Hemingway allows the story itself to stand as a symbol. Yes, it could be a metaphor for a writer being attacked by critics for a novel he slaved over, or for a starving farmer losing his lifesaving crop to a tax collector, but by not including any such symbols he invites you to come up with your own.
2) Make it as overt as you can
This is perhaps best illustrated by the short story The Second Bakery Attack by Haruki Murakami (you can read it here).
It's a story of a recently-married husband and wife who, in the middle of the night, suffer such hunger pangs that they are inspired to go out and try to rob a bakery. The story is intercut with the imagery of the husband (who narrates the story) in a boat, looking down at a volcano below the water.
This symbolism starts explicitly as the husband tries to explain the intense and supernatural nature of their hunger.
[...] when she said this to me, I began to think that this was a special hunger, not one that could be satisfied through the mere expedient of taking it to an all-night restaurant on the highway.
I can present it here in the form of a cinematic image.
One, I am in a little boat, floating on a quiet sea. Two, I look down, and in the water I see the peak of a volcano thrusting up from the ocean floor. Three, the peak seems pretty close to the water's surface, but just how close I cannot tell. Four, this is because the hypertransparency of the water interferes with the perception of distance.
As the story progresses, the husband returns occasionally to this image. Developing it together with the narrative.
While she hunted for more fragments of food, I leaned over the edge of my boat and looked down at the peak of the underwater volcano. The clarity of the ocean water all around the boat gave me an unsettled feeling, as if a hollow had opened somewhere behind my solar plexus--a hermetically sealed cavern that had neither entrance nor exit. Something about this weird sense of absence--this sense of the existential reality of non-existence--resembled the paralyzing fear you might feel when you climb to the very top of a high steeple. This connection between hunger and acrophobia was a new discovery for me.
Murakami is doing exactly the opposite of what Hemingway does. There is something he wants to express in painting this image of a husband and wife suffering inhuman hunger pangs. It's something quite deep and mysterious, so to elevate the story beyond just a simple telling of events, he pairs it with this entirely different thing.
The message seems to be that if we meditate on both elements, we may decode the story. Or perhaps we are simply meant to absorb the ideas, and let our subconscious do the rest. Either way, the story becomes something entirely different because of the inclusion of the symbolism.
I think this is exactly what Hemingway would call a symbol "arrived at beforehand and stuck in". Murakami has talked elsewhere about this being part of his process: whenever he tries to describe a certain pattern or phenomenon, he will come up with different things that are in some sense analogies, and see if he can add them to the story. In short, very conscious, reasoned symbolism.
The reason it's justified here is that the idea Murakami is trying to get at is likely much more unclear and mysterious than that of The Old Man a d the Sea. He needs to pair it with another perspective on the same thing, to convey to you that there even is something special going on. Without the symbolism, all we would have is just a story of a couple doing some slightly odd things.
I think your story probably fits best on the Hemingway end of the spectrum.
You want to show how Humphry changes as a result of meeting Cal. First moving from animosity to love, and then from love to madness. That's a very clear story, with a clear thrust, so extra symbolism isn't going to make it any clearer what you're trying to get across.
Instead, a good exercise may be to think of what kind of pattern of behavior the story itself might symbolize, and to see how you can emphasize that.
For instance, Humphry's initial animosity could be because he feels, on an instinctive level that this love is dangerous or frightening. This could be a like an alcoholic telling themselves they're not very social as a self-defense mechanism. Or perhaps the love he feels for Cal could have been wholesome and healthy, if he had been honest with himself from the start. This would be a bit like a closeted gay person denying themselves love, and then being very self-destructive and self-loathing about it when their restraints break.
Thinking for a bit about what the story could symbolize can help to figure out what beats to hit, and how to draw the characters' motivations more naturally. Like Hemingway, you can let all these different metaphors inspire you, and then leave them out of the story. Their job is to help the story itself become a symbol.