Hashirama knows he's not doing things right but he doesn't care, he can't allow Tobirama to marry someone or be with someone, is he selfish? Yes, that is, he is interrupting his brother's happiness, but every time he thinks about his brother as a couple it causes him discomfort to the point of turning his stomach, so each suitor is scared into never trying anything with her. his beloved tobirama.
Tobirama is his, only his, no one else has the right to be with him, no one is worthy of being with him, only him, only his anija.
Tobirama: - What are you thinking?
Hashirama: - nothing important, I'm just imagining
Tobirama: - Why am I leaving your side?
Hashirama opens his eyes in surprise and says nothing, Tobirama continues
Tobirama: Don't think I haven't noticed, you've scared every suitor... because you're afraid I'll walk away from you, or am I wrong?
Hashirama: -you're not wrong-
Tobirama:-•sigh• you shouldn't worry, I made a vow that I will only have a devotion towards you, that I would never leave your side
Hashirama: - I know, but I can't help it, every time they arrive I'm afraid that I'll walk away from you, you're mine, you know?
Tobirama:-•smiles• I know, see if this helps you, I myself will keep every suitor away from me, you know I will do anything for you
Hashirama: - Thank you, I think if it would help me calm down-
After all it's just a necessary protection in life, Tobirama doesn't need anyone other than his anija, only his, and Hashirama knows deep down that his brother would never break a promise.