Well, here goes:

It is getting too heavy for me.

I no longer use it and it is catching dust in the box on the shelf.

It looks too nice for me to shoot.

I cannot afford to shoot it anymore.

Good scopes are too expensive so I have to sell the gun.

I got it as a gift and I don't shoot.

I bought it on an impulse and having buyers remorse. Help! 
 
The hunt is more exciting/sweeter/more thrilling/etc than the kill I think is the root of all the reasons we give for wanting to sell.

The "hunt" being the idea and promise of procuring the next best thing. 

The "kill" being, "oh well, that's yet another one to add to the gun cabinet." 

"The grass is always greener......" might be another colloquialism at play here. We THINK that we need another gun cuz it'll be so much better than what we already have. So, we buy it, are excited about it for a little bit, and then decide to sell it when the excitement wears off. 
 
I don’t consider myself a collector although I do own a few more airguns than what I “need”. But I like to use the ones I do own. So, if there’s a gun I don’t find myself using almost at all, it will go. Then there’s the “I’m not drowning in money and can’t just buy & buy & buy”, so, as stated above, some will be sold to fund another. It’s like leasing a car, drive it for a while & then try another. I have bought stuff out of lust, and once I realize that I will likely not use it, will sell. 
 
So far I'm not seeing why these are excuses -- excuses implies that the natural state of things would be that we would keep our purchases forever. Whether we like them or not, outgrow them or not, need some cash or not, our hands or shoulders get too weak or not, whatever. Sounds legit to me. I just bought a gun, didn't marry it. And hard times can come to anyone.
 
also a story:

Along my competition archery years I have seen a large number of new members in the club also @ archerytalk forum.

They went to a store and shovel out 2-3-4 grands for latest and greatest, but after couple weeks the excitement is gone, the bow/arrow won't hit the "X" on its own.

Many of them didn't put enough effort into practice and training to learn the basics (tuning, aimin, holding, releasing, tuning and all over again), of course there it goes a brand new bow in classifieds...."not working" or "the next will be better".

Me and some other competition folks were shooting 2-3 hours/day at least tree times a week..at least thousand arrows every week... practice to get a muscle memory, to train for a subconscious routine. Without hard work won't work not even for natural talents.

I think with airguns same ...no better. Impulse buyers, not trained, not practising, no mechanically inclined but wannabe, and a worst is just hitting a next guy who will buy that rifle/pistol.

With these toys that is not enough having a large chunk of money in a pocket lets go buy something.

Have you seen those Youtube movies some rich guys just got a new Lambo or Bugatti or Ferrari (or call it whatever) and in five minutes he is ditching it at a very first corner.