Think of a camera shutter like a door.
Doors open and close, just like a shutter when it lets in light. Eventually the hinges that the doors are on will either break or become loose due to use or abuse.
Shutters that go past the shutter-expectancy do last for a quite a while after said expectancy but all shutters will fail after prolonged use.
Canon does replace the shutter but it does cost quite a lot (I think it cost about $150-$200).
150,000 shots is actually quite a lot. If I'm not mistaken you have a 7d because of that high shutter count.
I've had my T1i for about 3 years. I've shot for many different scenarios from low-light choir shows, to sideline danger zones for high school football games. The shutter life on a T1i is roughly 50,000 but my shutter count is only 29,765. (Magic Lantern said so...)
I wouldn't be worried at all. 150,000 is a very long life expectancy for your shutter. You are a lucky one haha.
Film SLRs do have a shutter life as well but I don't know how long they lasted or how to replace a shutter, even do I am an owner of a film SLR camera