The AI chooses which victory condition it's going for fairly early; possibly before the start of the game during galaxy setup. If it picks a conquest victory, then it'll get the 'we love war' diplo penalty. It's just there to prevent it deliberately preventing itself from winning by really, really liking people.
The actual amount of the penalty depends on personality type. The relevant xml can be found in Diplomaticmodifiers.xml in the Game folder, and is here:
<DiplomaticModifier>
<UniqueID>ConquestVictoryGoal</UniqueID>
<RelationEffect>-4</RelationEffect>
<AIPersonalityModifier>
<PersonalityTrait>Aggressive</PersonalityTrait>
<WeightModifier>1.25</WeightModifier>
</AIPersonalityModifier>
<AIPersonalityModifier>
<PersonalityTrait>Peaceful</PersonalityTrait>
<WeightModifier>0.25</WeightModifier>
</AIPersonalityModifier>
<Description>ConquestVictoryGoal_Dec</Description>
</DiplomaticModifier>
This means it has a base -4 per turn relations effect, increased to -5 for aggressives and reduced to -1 for peacefuls (who almost never pick a conquest victory if there's any alternative). This is modified by a couple of values in GalCivAIdefs.xml, and there's a diminishing return in effect so the more they dislike you already the less impact it has.
