It's neat that this scene demonstrates the ways in which, despite Tessa's relative youth, she's still experienced things that Magnus never has. Magnus is over 400 years old, so he thinks he understands how it works: you love someone, they die, you keep living. Tessa, on the other hand, is the one who knows what it's like to truly find your soulmate and to outlive them.
I think part of Tessa's different approach to love comes from the fact that she was welcomed into a found family while she was still a teenager. She met Will and Sophie and the rest as peers, even though she eventually watched them grow old in ways that she couldn't. Magnus, on the other hand, spent most of his youth with the Silent Brothers. He didn't start really interacting with mortal humans until he was already aware of his warlock nature, and so all of his relationships kind of left him as an outsider. Magnus was the mysterious stranger who blows into someone's life, or the powerful warlock they go to for help. He describes how even the people who lived with him until their deaths still saw him as something temporary, because they assumed that they must be temporary to him. This is why Alec is different, because he fully sees Magnus as an equal human partner. He welcomes Magnus into his friend group and into his family, because that's what you do with a boyfriend. For the first time, Magnus is being treated the same way Tessa was treated in 1878: as another messy person who needs friends.
It's just interesting to me how much the experience of being a warlock is shaped by how they're perceived. Tessa spent an entire mortal lifetime as part of a group of Shadowhunter friends. She was treated with unconditional love, and so she learned to give unconditional love in return. Because of this, she knows that the love she had for Will continues to live on long after his death. The love was part of her and part of the world, because they all accepted it without limitations. Magnus has never had that, and so he doesn't know how to approach a romance without thinking of the inevitable heartbreak. He's only ever been an immortal in love with a mortal, not a person in love with another person.