Because he is 20.
And because he is practically lodging in a house where another family is trying to live its life, with one member in particular enrolled in a course she is thinking of chucking in.
As for 'He's happily with his girlfriend & she's happy too', you should look up dopamine and its effects.
Also check out signs that a relationship is not healthy - a relationship quickly becoming serious is one.
www.zensensa.com/eight-red-flags-in-dating-you-should-not-ignore/
Intensity
We get sucked in because they create high emotional intensity, and it’s often fast and feels magical.
Like we are a perfect fit.
Sentences like “I have never loved anything like this before” and talk about marriage, kids, and moving in together often happen quickly.
Emotions make us forget and miss warning signs, so you must slow down no matter how good it feels and understand you can’t make good decisions in this state of mind.
You are in a chemical obsession, and your logic is offline.
You also project your stories and fantasies onto this person, and we have no idea who this person is or how being in a long-term relationship will be like with them.
Don’t buy into the emotional intensity.
Love is slow, consistent, and stable.
What comes quickly goes quick, and like a drug, when you go high fast, it will also come down hard fast.
If you tend to be drawn to high intensity, it could be because you are anxious, attached, or co-dependent and the quick commitments and intensity make you feel safe that this person will not abandon you.
The irony is that these relationships are more unstable and less likely to last.
If you are going to spend your life together, then what is the rush?
We tend to put forward our best sides initially, so you don’t know someone after dating a few months.
Emotions are good at telling us what we need right now, but they are not good at telling us what is good for us long term.
If it’s very intense and moves very fast, then it’s a big red flag.
(Fwiw - we are all commenting on this, days after the OP has left.)