Olas said:
I just don't understand what's to stop me from doing this non-stop for hours until I have a super leveled up character.
Exponentially growing level cost. The more you level, the more expensive it is to level.
That said, farming/grinding is a part of most RPGs and there aren't many RPGs where you couldn't do the same. There's also the fact that you don't gain THAT much by leveling after a while, certain points with certain skills just stop being effective (Endurance only gives you Stamina until 40, Dexterity gives you a pathetic increase in damage after 40 as well etc.)
The point you're at, the dragon bridge only gives you 550 souls for each reset, which, while awesomely beneficial to do a few times (usually lets you level up a few needed skills), becomes pointless and futile very soon. I only do it a few times to get my Strength to 16 (or was it 14?) for the Drake Sword (yes, I know it sucks, but at that point it is awesome and I need that much Strength or more later anyway) and maybe some Endurance points to stop myself from getting overweight with gear and then move on. After a certain level (very low), it takes a lot of runs to get even a single level out of it.
Soul farming is rather useless for the most part, you usually want to farm something else WHILE getting souls. There are two places you'll want to farm. One is beneath the Firelink Shrine past the ghosts (you have to get through it once to drain the water) where the mobs drop the upgrade stones - it's a fast run, nets you a decent amount of souls and a fastastic drop rate for upgrading your weapons (if you're doing it right, ie. 10 held humanity+the drop rate ring) and the mobs are remarkably easy to parry/riposte (or backstab, but riposte kills faster). The other place is in the DLC near the end where you can farm the Humanities to your heart's content.
As for farming souls, if you really want to do it, PvP is your friend. Particularly the Cat Covenant PvP as it has (or at least had, I imagine a lot of players moved on to DS2) an excellent rate of PvP and you're always invading above your level. While this might seem hard, the fact is that you're gonna run into clueless players who've leveled their characters to ridiculous levels, which is gonna net you a LOT of souls. After a few days of PvPing there, I had a few million souls on me. The way PvP works is, you get a percentage of souls that cost your opponent to reach the level they're at. So when you run into a ~712 player who thought it'd be a great idea to waste time leveling up like crazy despite it giving him very little advantage, if you kill him (and it's not at all hard most of the time since those players are mostly awful) or have one of your partners kill him, you'll get some 800.000 souls just for him, because that's what his final level cost him.