Behavior-wise, it's best for a customer to purchase again for their own purpose. But not all merchandise can sell that way, especially with big-ticket items like dining tables, computers, or blenders. Sometimes you'll get repeat purchases for them but it'll be rare.
Selling gift cards might be an option. If your customers love your products but don't need to buy frequently, you can offer gift cards they buy for loved ones. This acts like a stronger form of word-of-mouth advertising.
Shopify includes support for gift cards, including some advanced features (e.g. free cards as incentives, gift card discounts) though not all themes support everything yet.
Taking the time to get them setup or updating your theme to support them is a great use of time before the winter holiday season.
Eric Davis
Customer behavior analysis for better Shopify store performance
The Shopify App that increases repeat customer purchases through customer behavior analysis.