Chaim > chasdim just do it out of fear and the chumras come first.
I have an idea about the chumros – why so many people run for chumros bein adam l’makom, but not always bein adam l’chavero. As you say – fear.
You have a relationship with Hashem and you have only limited information about what He expects from you. What if $50 lulav is not enough for a chashuve Yid like you? My neighbor who is an am haaretz spent $100, so it may be $200 is expected from me?! This is a Jewish version of Pascal wager : if your error on being machmir costs you $100, and your error on meikel costs you olam habo, the choice is obvious.
It looks easier when you are dealing with people who provide you with feedback.
Someone is selling a car. You say – how about $2000 for this clunker? The guy agrees. So, obviously the price was right.
Your wife asks – when are you going to be home? After the latest maariv in town and a class after that. OK, says the wife … obviously, she is happy that you are learning and she has the zechus to put kids to sleep.
There two possible errors here:
1) it might be that Hashem did not really care about $200 for the esrog. OK, so the loss is just $150, right? No, the loss will be all the time and effort you spent pursuing these chumros while neglecting other issues and learning and maybe stealing more from others for the chumros (if yo are meikel on using OPM, other people’s money).
2) you can’t always rely on getting feedback from people.
a) They may think that you are dishonest and rude person, but they will not tell you that in your face. Did you have hear people saying bad things about others? Do they usually say it in their face or only behind their back?
b) they may know that they can’t expect more from you. A wife would be the first to know that.
c) they may not understand themselves at the moment that you are not doing enough for them. They may not know choshen mishpat to know, for example, that you have to disclose problems with your clunker when you sell it, or that the buyer is not allowed to insist on a lower price same way as the seller is not allowed to insist on a higher …
So, bottom line is that one should appreciate complexity of human psychology and be fearful that they are not machmir enough in bein adam l’chavero , that are so machmir that Hashem will not forgive them until people do ..