If you are susceptible to something such as infections or earaches, it means you are likely to become sick with these things.
Have you ever received something you don't want? Well, with susceptible meaning "likely to be influenced or affected by" that is probably going to be the case. If you're susceptible to flattery, and someone wants something from you, all they have to do is give you a compliment or two and you'll do what they want. Material that's susceptible to cracking won't be in good condition for long.