Oh I hate this words. This sentence might come if someone is expecting something. Like if you don't, they won't be disappointed. But if you do, they'll be sad. Simple.
In my eyes, expectation is closely related to hope. But hope is just like a two sides of coin, it can be good, when hope gives you motivation to reach your goal, but in the other side, hope a.k.a expectation might kill you when you get disappointed as your hope is not fulfilled.