Different races like/dislike certain others. Prior to UCP 5.9, all races were neutral toward you if you played a custom race, but now it uses the disposition settings for the race portrait you choose. Here's the disposition table for 5X:
https://imgur.com/a/GGwmQvS
The way it's formatted, the race at the top feels this way about the race on the left, so the Silicoids feel neutral toward the Terrans, but the Terrans feel uneasy toward the Silicoids.

The basic structure is as follows:

First we have the 8 races Alkari, Bulrathi, Darlok, Klackon, Meklar, Mrrshan, Sakkra, and Silicoid. Each of these is restless toward one of the others, e.g. the Alkari and Mrrshan hate each other, and relaxed toward one other, e.g. the Alkari and Sakkra like each other. Each race feels wary toward their enemy's relaxed partner, so Mrrshan and Sakkra are wary to each other. Thus they form a set of 4 races in 2v2 teams. Each team slightly prefers one of the other pairs in the other 4-block, so the alkari are uneasy about the silicoid and meklar (who are allies with each other), and the alkari are wary toward their enemies, the Darlok and Klackon.

So these 8 races each have a symmetric relaxed relationship with one other race, an uneasy relationship with 2 more, a wary relationship with 3 more, and a restless one with one more.

Add in now the Psilons and Humans. They get along with most races, with the Humans having mostly relaxes relationships and the Psilons having mostly neutral relationships. Here there are some asymmetries though, like the Klackons are relaxed toward the Humans, but the Humans are uneasy toward the Klackons. These asymmetries add a bit of chaos to the otherwise symmetric, predictable structure above.

The DLC races add more complications, with the Terrans disliking and being disliked by most (Humans in particular are mutually restless with the Terrans), and the Gnolams are liked by most and feel neutral toward most others, with the Elerian and Trilarians being naturally allies that have various preferences among the other races.

In general, you see a lot more "unease" and "wary" than "neutral" and "relaxed." This is to make diplomacy more challenging in general and conflicts inevitable. Having this table is nice though since you can always make sure your "natural ally" is in the game with you.

The way I came up with who likes who is partially lore, and partially just me trying to come up with a good balance. Honorable mention to the Mrrshan, who hate the Trilarians 'cause cats hate water.