Hi
It is difficult to opine as to the necessity of a root canal treatment without a clinical examination and an xray. Your dentist has evaluated you and suggested the course of action based upon what he found. The only option that you may exercise if you are not certain of the proposed plan is to take a second opinion from another dentist.
Next Steps
If the sensitivity is mild, and lasts only while the inciting stimulus (e.g. cold water) is in the mouth, subsiding spontaneously after its removal, you can ask your dentist about the feasibility of alternative treatments, such as pulp capping.
You can also get a second opinion on the tooth. However, these need to be done, and the appropriate treatment completed as early as possible for any possibility of success, lest the decay should become more extensive than it already is.
Health Tips
Be careful while discussing pulp capping as an option with the dentist, and understand that you might still have to get a root canal treatment at a later date.