I think Eckhart Tolle's books can be a little fluffy, but he's got some interesting ideas I can't dismiss. Here's one that struck me (Link):
Most people treat the present moment as if it were an obstacle that they need to overcome. Since the present moment is life itself, it is an insane way to live.
We do this all the time. We're looking to the future at the expense of the present. "If I can just get past this meeting/project/problem, my day will get better." We spend so much time in the past or in possible futures. They're illusions. They're not real. The present is real. It's the only thing that's real. But we don't spend much time in our minds here. We're always looking back or looking forward. We're not living when we're doing that. We're dreaming. And we're missing out.
I have a friend whose second daughter was born a few weeks ago. He and his wife think two are enough, so this is his last kid. This is the last time he'll have a newborn in the house. One way to frame it is, "This is the last time he has to go through dealing with a newborn." Another is, "This is the last time he gets to experience a newborn."
These two sentences are very similar, but have noticeably different perspectives. One tilts negative, the other tilts positive. "Has to" vs "Gets to". How we frame our lives in our minds has an incredible impact on how we ultimately perceive our present moments.
Patton Oswalt calls it, "Revel hard."
Confirmation bias plays a role here. We seek out confirming evidence of our assumptions. Put another way, if we assume things will be positive (or negative), we'll be on the lookout for evidence to confirm it. We'll see our reality as positive or negative. Our expectations have a massive impact on our life experience.
This is usually where the self-help books stop. They explain confirmation bias, tell us to think positive, and our lives will be better. There's a nuance there that took me several years to figure out. It's not positive. It's neutral.
Think neutral. Keep the "I get to experience this". But stop there. Don't add, "...and it's going to be great." It's enough to experience it. We get into trouble when we start assigning value statements to events that weren't trying to impress us in the first place.
reality - expectations = happiness.
I didn't come up with it, but I love this little math expression. If I constantly have high expectations, I'm likely to be constantly disappointed. Reality isn't that amazing all the time. The mistake is to swing too far in the other direction and think, "if I have low expectations, then I'll be pleasantly surprised all the time, which means I'll be happy." False. Remember, we seek out confirming evidence. If we look hard enough for something, chances are we'll find it ... one way or another. If we walk around with low expectations, we'll find reasons for them. "I think this is going to suck, and look, I was right. It sucks." We forget about the happiness equation and fixate on why things are so terrible. It's a drain and nobody wants to circle drains.
Few or no expectations is the key. Just take them out of the equation, and then "reality = happiness".