I'd like to add a little bit, though I haven't played Endless Space 2, and don't know how exactly it all works yet. This is more a theoretical understanding based on real life;

In real life, movement through space is reliant on things like fuel, mass, engine efficiency, etc. A number known as delta-v or ∆v is useful in figuring out how far or where you can go. In essence it's a measure of how much velocity you can "spend" to get around. An example: SpaceX has just designed a new rocket with a ∆v of 30km/s. To get into orbit costs this rocket 9km/s. To reach Mars costs an additional 0.5km/s. To reach Pluto instead might cost an additional 3km/s, and Mercury perhaps 20km/s.

Now, suppose you've set course for Mercury - you've used up 29km/s of ∆v out of 30km/s total. On the way there another space-rocket which belongs to Blue Origin is leaving Mercury and you will pass right by each other. You'd like to stop and have a chat with them for a few days, board each others' ships, have a spot of tea. Unfortunately, you've already spent your ∆v allotment, you can't change course, you only have 1km/s left. You'd have to spend another 20km/s to cancel out your current velocity, another 20km/s just to roughly match Blue Origin for a few days, and then another 40km/s again just to start going back to Mercury. A total expenditure of 109km/s.

Hopefully you can see some theoretical reasons why it's not as easy as just 'turning the car around' to meet with people flying past you at ludicrous speed.

With that said, if you decided to throw a rock out the window of your spaceship with the intent of hitting the window of the other spaceship so they can look out and say hi, the combined velocities means that rock hits at a speed of ~40km/s, and delivers roughly the energy of 170 kilograms of TNT. So there's potential for some gentlemanly rock throwing going on in some sort of meeting engagement like this, it's just that you'd both probably die.

Now, being that Endless Space is a videogame, none of this applies.

But it's neat to think about.