IMHO, the short answer is redundancy.

Both 想 & 要 in this context represent or express a desire that a certain event occurs, all having had its origin as a thought process.

想 by itself is a thought process per se, i.e. the "mental root" of all your desired actions.

要, in this context is a resultative desire which also has its origin from a thought process, i.e. 想; it cannot be otherwise.

Thus you find your friend's advice of "...that I can only say 一定想 or 一定要 but not 一定想要.

So, you use one or the other and not both to avoid a redundancy.

However, there is a song 原来你什么都不想要, https://www.youtube.com/watch?v=np54VFHtA6w

The issue is therefore a lot more complex than meets the eye.

IMHO, the short answer is redundancy.

Both 想 & 要 in this context represent or express a desire that a certain event occurs, all having had its origin as a thought process.

想 by itself is a thought process per se, i.e. the "mental root" of all your desired actions.

要, in this context is a resultative desire which also has its origin from a thought process, i.e. 想; it cannot be otherwise.

Thus you find your friend's advice of "...that I can only say 一定想 or 一定要 but not 一定想要.

So, you use one or the other and not both to avoid a redundancy.

However, there is a song 原来你什么都不想要, https://www.youtube.com/watch?v=np54VFHtA6w

The issue is therefore a lot more complex than meets the eye.

@Becky's answer in another post is very helpful here.

IMHO, the short answer is redundancy.

Both 想 & 要 in this context represent or express a desire that a certain event occurs, all having had its origin as a thought process.

想 by itself is a thought process per se, i.e. the "mental root" of all your desired actions.

要, in this context is a resultative desire which also has its origin from a thought process, i.e. 想; it cannot be otherwise.

Thus you find your friend's advice of "...that I can only say 一定想 or 一定要 but not 一定想要.

So, you use one or the other and not both to avoid a redundancy.

IMHO, the short answer is redundancy.

Both 想 & 要 in this context represent or express a desire that a certain event occurs, all having had its origin as a thought process.

想 by itself is a thought process per se, i.e. the "mental root" of all your desired actions.

要, in this context is a resultative desire which also has its origin from a thought process, i.e. 想; it cannot be otherwise.

Thus you find your friend's advice of "...that I can only say 一定想 or 一定要 but not 一定想要.

So, you use one or the other and not both to avoid a redundancy.

However, there is a song 原来你什么都不想要, https://www.youtube.com/watch?v=np54VFHtA6w

The issue is therefore a lot more complex than meets the eye.