Why does he fall in love with a different lady every episode?

Read the books. He had many.

Never seen this show...
But im gonna take a guess here and say hes a guy
So...
Guys are hot for new sexy girls
True talk

I don't get your criticism seeing how he DOES NOT fall in love with a different lady every episode. More like once or twice a season.

Because he’s Casanova.

Does he, though...? Implicit here is that he's falling in love too quickly with too many women. I'm assuming the complaint is that, for the romances to mean something, we can't think they have no connection.

But is he doing that? Let's take a look, shall we?

In episode one, he has an established relationship with Brida. They grow up together, they're childhood friends and sweethearts. I think this is believable. As Uthred hangs out with Saxons more, Brida and he drift apart.

In episode four, he is set up with Mildreth. It's an arranged marriage for political reasons. He thinks she's hot at first, and his natural decency makes him treat her nicely. Over the course of the show, we will clearly see he never really falls in love with Mildreth. There is a "time passes" here, too. They're together for awhile.

In episode six, he meets Iseult, the witchy woman who he falls for pretty fast. I think this is his pagan leanings giving him a nostalgic yearning combined with Iseult's general attractiveness and their shared worldviews. Still, he doesn't leap right into bed with her.

Season two: time has passed (not a bunch), and over the course of the season, Uthred gets into a relationship with Gisela. This one isn't that sudden, and comes after a passage of time.

Season three: minor spoilers, but after his relationship with Gisela is ended, Skade comes into the picture. I think it's safe to say that any attraction here is definitely not love. Maybe some heat, but Uthred and Skade are trying to out-use each other and the way it ends, well...it wasn't love.

Seasons three and four: Uthred and Aethelflead get a thing going, but again, it is slowly brought in and established. This one takes the longest of any relationship build.

We have six relationships here. He only seems to really fall in love with Brida, Gisela, and Aethelflead, and barely shows anything real towards Skade. Iseult is arguably a too-hot-too-fast relationship. Mildreth isn't that important either.

His character was a warrior in his late teens.

Tldr - because nature