Choosing to charge interest or not depends largely on your specific situation. If you are lending money to a family member or friend, you may feel uneasy about charging them interest and profiting from the transaction. As a lender, you are under no obligation to charge interest.
However, choosing to charge interest can also come with its benefits. Besides allowing you to make some money, charging interest can incentivise a borrower to make payments on time and pay back the loan within the designated time. Keep in mind that there can be limits on interest rates.
In Singapore, when you charge interest to a borrower, there is a rebuttable assumption that you are a moneylender. In this case, the restrictions set out in the Moneylenders Act 2008 and Moneylenders Rules 2009 will apply to you. This legislation puts restrictions on moneylenders, such as maximum allowable interest rates, as well as requirements, such as providing statements of account, receipts, and other documents. If you are or think you might be a moneylender, consult with the applicable legislation for the restrictions and requirements that may apply to you.