Where are the aliens?

So I've come up with a few possible reasons why we haven't seen any aliens yet, despite the ridiculous amounts of time they've had to get here.

1. There aren't any. Intelligent life realises how pointless existence is, and dies out. Evolution keeps intelligence levels just below this critical level until their resources are exhausted, and they die out without migrating to other planets.
2. Interstellar travel is impossible. The energy cost is just too high, or interstellar radiation destroys organized molecules before they get anywhere.
3. Interstellar travel is too slow. Travelling 1000 light years (1/100th of the galaxy's diameter) at 1% of lightspeed takes 100000 years, and when you get to the next habitable system you have to spend some time setting up (at least) fuel refineries etc. Intergalactic travel is right out.
4. They know we're here, and they're leaving us alone. Not much else to say about this one.
5. We're the first. Life just hasn't appeared anywhere else, because conditions are not right on other planets.