Currently, I am learning Portuguese. I have some knowledge of Spanish as well.
The biggest difference in conjugation (indicative present tense) that I have found between Spanish and Portuguese is that Portuguese does not go through stem change (at least not that often) than Spanish. In Spanish, it is very usual that we find that we have to change e ⟶ i, o ⟶ ue, etc. When we conjugate, however, Portuguese does not have it, except in some cases. Why is that ?
Even though Portuguese does not go through stem changes so often as Spanish does, there are still cases when it happen, especially in first person singular (Eu). In some cases, the change is only to preserve the pronunciation of the infinitive form (Ex: Fugir ⟶ Eu fujo). While in other cases, it kind of works like Spanish (Ex: Preferir ⟶ Eu prefiro; but você prefere, Dormir ⟶ Eu durmo; but você dorme).
So why in these cases, there will be stem changes in Portuguese but not for another pronouns. Is there a reason behind?