Like an old married couple

[wellplayedpenny]

"There is no “dance” in my opinion.
They go from one crisis into the next.
They argue, they insult each other, they have sex….and than the whole thing starts again.
For me it is a primitive affair without human depth or warmth. Not funny and not romantic."

I think this all ties into the fact that Penny and Leonard have nothing in common with each other. It's been pointed out several times on the show and instead of giving some plausible explanation or a sincere attempt on both Leonard and Penny's part to do something about it, their lack of connection is just shrugged off as 'twoo wuv' and that's that. Something that makes little sense since Leonard's latest infidelity makes Penny seem like an idiot for staying. Being with someone only because you love them is something teenagey-angsty but when that person goes on to cheat on you and you still stay--that says time in a therapist's office.