Usually, a personal loan is granted without needing a guarantor, co-signer, or co-applicant. In effect, the waiver of any requirement of a guarantor is one of the characteristics of a personal loan. Typically, lenders ask for guarantors for larger ticket loans such as home loans, loan against property etc. This makes a personal loan less cumbersome to get. It is after all, difficult and time-consuming to get someone to agree to be a guarantor. This feature of a personal loan is one of its selling points apart from the fact that personal loans can be used for practically any purpose. However, some lenders may ask for a guarantor for a personal loan if the applicant has a low credit score.
For more information on guarantors, read this thread on guarantors