It's not your job to keep their spirit up.
But it's not necessary your job either to bring them down.
Just tell them honestly if they take a big risk that you see a risk and why.
Ask questions!
I’ve got a bunch of friends in the same boat…Poorly thought out plans become pretty evident with simple questions like “what’s your plan?” Or “do you know how to build that product?”
But idk the other day I had a conversation like this:
Friend: “In a couple years I’ll be making over $1M/yr!”
Me: “How are you gonna do that?”
Friend: “I’ll just make more sales. Why can’t I?”
Some people are just delusional and I change the topic.
This is exactly like the book "the mom test" but for once we are the mom.
If it is obvious that your friends want support, give them that. Unless they are betting their life savings or doing something extremely risky why bring them down.
If you know them, they know you too, and they are sharing with you for a reason. But if you’re unsure how best to support them - ask this:
“Would you like me you think through potential risks and pitfalls, or are you looking for emotional support?”
If they answer the former, go right ahead and challenge them. If the latter then share your hopes for their success. Sometimes they know they have blind spots and they need your help before investing too much effort in the idea, sometimes they just want to share their enthusiasm.
You have the right mindset. Being supportive of someone doesn't mean you have to be a sycophant and start doing all the song and dance. In fact, these people are the exact antithesis of being supportive, they usually become the catalyst for toxic narcissism and impending doom at some point in life of the people they support. In fact, the most supportive person is someone who speaks their mind out and gives genuine advice they need to hear, not the one they want to hear!
Just remind yourself that these people have probably thought harder about their problem domains than you have.
Just make sure they are risking their own skins, not others, and then provide encouraging words if you don't believe it.
Try a "both sides" approach.
Here's the cool thing, here's the drawback.
I think you should think about it in terms of how much “naive optimism” you want to foster in your friends.
In truth, there’s good reasons do doubt every project, even the ones that end up being highly succesfull.
Unfortunately, it seems like human beings are highly pulled towards either working on something with unrealistic confidence, or sitting on the the couch and watching netflix.
Working on something with unrealistic confidence, even if that project itself is completely doomed to failure, is probably better for you than watching Netflix.
The one exception is, there is a higher than average probability that startup people with naive optimism end up getting into a position where they decide to end their life.