Well, are we talking realistic space travel here, or are we assuming we have Star Trek style technology available?
If we're talking real tech, at least the stuff we have that is definitely in the works and the stuff that we know will eventually work...well, we've got the whole problem about how space travel doesn't work like you think it does. Right now it costs a couple (exact amount escapes me at the moment0 hundred grand, just to launch a single ton of material into space, which means all those images we've grown up with that show space vessels looking mostly like a contained cruiseship, well, those are wrong. Sad truth is that you'll likely be floating around with wires hanging out everywhere, in cramped spaces. Also, you'll be shipped with a small amount of water that you'll have to recycle on the ship, so yes, you'll essentially be living out that scene from Waterworld where Kevin Costner filters his own urine then drinks it. This basically means that we won't be shipping out anywhere that won't benefit us in some way, whether that be colonizing or stripping another world of resources, there simply has to be some upside to the cost it will take to send a ship out into space, and no, it won't be to "explore new worlds and civilizations" we have unmanned probes for that that will be far more resource effective and exceedingly more viable to send out for such things.
Also, unless we make some pretty huge advances in being able to achieve and escape orbit, then that planet you got sent to colonize will either have to be cleared of any obstacles to it's growth and survival (i.e. any sentient or native lifeforms that may interfere) or you'll die with your fellow colonists on some distant chunk of rock. So yeah, there's that awful horror of being stuck on some hell world until your people manage to build that starport/space elevator/space gun/whatever the eff we're using to launch our ships into space at that point.
Finally, when it comes to aliens, well, to put it bluntly, unless we can bone them and produce viable offspring, then one of us will eventually have to wipe out the other. It's just survival of the fittest, two intelligent and sentient species that are incompatible with one another simply cannot live together without one eventually dominating and eliminating the other. Look at the Neanderthal. And yes, even if it's some freaky fish monster that can only live in a ammonia/nitrogen atmosphere, they'll still likely be trying to harvest the same worlds and varied space junk for the same resources we will, and will maybe even be trying to terraform the same planets.
If we've got the Star Trek stuff, then yeah, we'll probably be all peaceful. We'll just need to terraform a few worlds just to keep up with our growing population, but we'll have magical goodie bags that will be able to transform anything we put into them into any other goods or materials we need, hell, we won't even need to farm or anything because we can just tap a few keys and make a complete meal appear out of thin air.
