Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Drain.
Melt butter in a skillet over medium heat. Add celery and onion and saute until soft, about 5 minutes. Add garlic and saute until fragrant, about 1 minute, being careful it doesn't burn.
Mix soy sauce and sugar together in a small bowl; stir in to the vegetable mixture. Immediately add drained spaghetti and cook until heated through, 2 to 3 minutes. Serve hot.