A lot of the issues people have seem to be coming back to the manner in which players go about changing or tailoring lists. The fact of the matter is, in a small, tight-knit group of players if no one ever changed their lists then the game would stagnate and potentially become very boring indeed.
Surely it is natural to want to win at a war game? Isn't that precisely the point? If you just want to have fun with friends then go watch a couple of epesodes of Archer or Arrested Development, or even some funny cat videos on youtube. By playing 40k or any other sort of game for that matter, there's the added element of competition. And that's where it's possible for problems to occur, because for some people, with terrible social skills, the competition can take over their entire experience and this can sour the experience for others.
Be competitive, change your list, try and kick as much ass as you can possibly manage, and enjoy it. Don't try and play inside other players' heads, countering their every move and unit choice, doing this will leave you wide open to some very easy counters yourself. Instead, take well thought out, logical steps to give you the best chance at beating your opponent. You might win convincingly one week, but the choices you made should encourage your opponent to be better prepared themselves next time.
My Complete DA collection
That's me breathing fire in my avatar, my aim was a bit off to start but i got all those orks in the end!