Usually when someone initiates a conversation, they may have given the topic some thought, possibly even come up with a few potential scenarios for discussion.

Other parties invited into the conversation may or may not have thought about the topic, and therefore may be starting from scratch.

Sometimes it may be helpful to introduce your topic, ask if the person has thoughts on the topic, ask if this is a good time to discuss, and/or share the due date or deadline for decision-making and plan ahead to discuss before the final decision is due. To help spark thinking, you might share the thoughts or scenarios which have occurred to you. The idea is to role-model thinking/planning/decision making processes without imposing undue pressure, and to encourage and affirm the child sharing his/her thoughts.

While some teens may be quite self-absorbed, others' minds may be busy thinking about big concepts, and therefore rarely focus on themselves and what they like. If this might apply, one idea to encourage a healthy degree of positive focus on personal preferences, is creating lists or collections of favorite things in a pretty notebook, journal, sketchpad, or as photos on a smart phone: a personal scrapbook version of pinterest, free from the scrutiny of others. To role model this, you might also make collections yourself, and/or have all family members make collections and have time for sharing whatever a person wishes to share from their collections. A fun conversation starter: What do you like about it?

Having general conversations with open-ended questions about material things in the physical world such as vacation destinations, clothing styles, shoes, accessories, colors... may help guide her to think about what she likes, without feeling like she is under a microscope to provide an answer. For example, when shopping in stores or online, you might wonder aloud:
- How does the Pantone color-of-the-year (2019 = Living Coral) compare with padparadscha, with millennial pink? (Do you notice a lot of items in these colors, recently?)
- If your kiddo likes to cook or bake: What do you think of the current colors of the Kitchen-Aid stand mixers? (Do you remember the colors offered a few years ago?)

Conversations about entertainers or professional athletes, their volunteerism, and the causes they support may also help kids realize what they like, value, and feel affinity for.

In my observation and experience, this helps build a bridge to successful decision-making.

ETA:
When kiddos don't know what they want for dinner, here are a few approaches which have worked to guide the child (YMMV):
- Option 1: Parent has created a blank menu form which suggests what a healthy meal consists of and provides fill-in-the-blank spaces for the child to complete: Produce (Veggie 1, Veggie 2, Fruit 1, Fruit 2), Protein (Eggs, Cheese, Fish, Meat), Carbs (Pasta, Potato, Rice, Bread/roll/biscuit), etc. If kiddo goes through the effort of completing the menu, s/he gets that meal as soon as practical... usually this night or next.
- Option 2: Child chooses "Sampler Plate" and parent provides a plate with little bits of many foods to taste.
- Option 3: (Default) The old-fashioned you-get-what-we're-serving and your only choice is take-it-or-leave-it.

Child learns that their investment of thought and effort pays off. On the other hand, some days they really don't have a preference and anything is fine, appreciated, and eaten.