I am writing a detective novel and wanted to include two best friends who are working together. Eventually they fall in love but one of my readers said it's too boring and cliché. Is that true? Should I avoid this?
Yes, it is a cliché. I can list in my mind more than ten instances where work colleagues fall in love.
But if you are willing to avoid a cliché, I guess you are without luck. If you have them not being a couple, this is another cliché. If you have them in a platonic relationship, this is another one.
As TV Tropes clearly show, there are a lot of variations that would fall into one or other cliché (Warning! Tvtrope link: https://tvtropes.org/pmwiki/pmwiki.php/Main/JustFriends):
- Official Couple
- Just Friends
- Like Brother and Sister
- Belligerent Sexual Tension
- I Don't Want to Ruin Our Friendship
- Mistaken for Romance
- Friends with Benefits
- Platonic Life-Partners
- Strictly Professional Relationship
And many more. As a disclaimer, they do differentiate between a trope and a cliché. You are the one to judge if the way you use it is boring or not.
So you need to decide what kind of story would you like to tell. If your main goal is to avoid clichés, then you are in a tough position.
Of course it's a cliché, everything is!
But that doesn't mean clichés are always boring and to be avoided at all costs. What makes a cliché boring is when an author copy-pastes it without bothering to make it their own. That's the key. There's a reason clichés are, well, cliché. It's because they work. If they didn't, no one would bother using them. It's just that they've been used so often that people get tired of seeing the exact same story everywhere they look. All you have to do is try to make it an original cliché. So if the romance between your characters is an important part of the story, then you shouldn't try to avoid it. You should try to make it original enough not to be boring.
So basically, don't follow the mold too closely. Put your own spin on the cliché and make the characters well rounded and interesting, and people will hopefully enjoy it. Obviously, though, you can't please everyone. Some people will dislike the 'friends become lovers' cliché/trope no matter how well or differently it's done, while some will like it no matter how derivative!
Yes, it is a cliché and yes you should probably avoid it. Even if you think it has happened in real life.
In a way, it is implausible if they are BEST friends, that implies they have known each other a fairly long time, and therefore if they were sexually compatible in the first place, they probably would have explored that already. To make it happen they would have to already have the sexual attraction, and be prohibited from acting on it for some reason. One is in a committed relationship, or perhaps their love is forbidden (homosexual, or crosses social or cultural boundaries; e.g. a workplace friendship, a military friendship between an officer and a non-officer). In such cases, unrequited sexual attraction might be acted upon when the situation changes, due to a change of status, place, etc.
Fairly new friendships, say within a year, can develop into love; it is not unusual for sexual attraction to be not acted upon for that long, and sometimes sexual relationships begun in mutual lust can develop overtime into love.
But psychologically speaking, best friends that know each other well are unlikely to choose to change into lovers; if it was going to happen, it would have already happened, because they have had plenty of opportunity to find something they romantically love in the other person, and would have felt sexually attracted to them and likely broached the subject already. Romantic love demands physical attraction. Love for siblings, parents, friends, children does not, but that is not what we call "falling in love." And physical attraction, although it does not have to be instantaneous, occurs fairly quickly after meeting and interacting with someone, and we expect it to either happen soon or never. It is not the kind of thing that occurs years after you've known someone (unless those years were as children).
If you want them to fall in love, I'd make them more like classmates or coworkers that have previously had little contact with each other, and the detective story ties them together on an adventure, where they get to know each other and because of that fall in love.