Well, for me it's different for different Shepards. Here are a couple of mine.
Selyn (F) - Colonist, War Hero, Paragon - Synthesis
She's an idealist who has long believed that intolerance, corrupt authorities, and general lack of communicationa and understanding are at the root of most conflicts in the universe. She chooses Synthesis because she doesn't think she (or an AI based on her) should necessarily be in control of the Reapers and is not willing to sacrifice EDI and the geth. She doesn't see Synthesis as compromising diversity, but as enabling people to make better choices and achieve deeper understanding through their increased abilities to accumulate knowledge. She also (headcanon alert) gains enough control over the process before "dissolving" to ensure that it happens in a way that's reversible for anyone who doesn't want it.
Kane (M) - Colonist, Ruthless, Paragade - Control
This guy might be the most idiosyncratic of my Shepards, and definitely very unlike myself IRL. He actually does have some strong principles (generally alien-friendly and multilateralist, for example), but he's also somewhat cynical about the possibility of persuading everyone of their value. He didn't actually murder any surrendering Batarians at Torfan, but let the rumor circulate anyway in the hopes that he could use it to his advantage, and similarly sometimes chooses to bluster and intimidate his way through conflicts. At the same time, he cured the genophage, brokered peace on Rannoch (though with the "I will stand aside and watch them lay you to waste" dialogue option), saved the rachni, etc. He wants to give everybody an equal chance at a better future when the war is over, but he also thinks it might not be such a bad thing to have a leader who can drop the hammer if things get out of hand. He still leans Paragon enough that he got the Paragon Control narration at the end, which generally fits his character.